The documentation for org-complete-tags-always-offer-all-agenda-tags
currently reads:

"If non-nil, always offer completion for all tags of all agenda files.
Instead of customizing this variable directly, you might want to
set it locally for capture buffers, because there no list of
tags in that file can be created dynamically (there are none)."

It appears the second sentence is missing a few words.

"...Instead of customizing this variable directly, you might want to
set it locally for capture buffers, because there *is* no list of
tags in that file *that* can be created dynamically (there are none)."

That aside, the wording of the second sentence could be clearer.
My humble suggestion:

"If non-nil, always offer completion for all tags of all agenda files.
Setting this variable locally allows for dynamic generation of tag
completions in capture buffers."

Thanks for your time,
no wayman


Emacs  : GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.1)
 of 2018-09-24
Package: Org mode version 9.1.14 (9.1.14-1-g4931fc-elpaplus @ /home/n/.emacs.d/elpa/org-plus-contrib-20180917/)

current state:
==============
(setq
 org-duration-format '(("h") (special . 2))
 org-src-mode-hook '(doom-modeline-set-special-modeline org-src-babel-configure-edit-buffer
                     org-src-mode-configure-edit-buffer)
 org-after-todo-state-change-hook '(org-clock-out-if-current)
 org-metadown-hook '(org-babel-pop-to-session-maybe)
 org-clock-out-hook '(org-clock-remove-empty-clock-drawer)
 org-refile-targets '((org-agenda-files :tag . "org-agenda-files")
                      (org-agenda-files :maxlevel . 20) (my/org-files-list :maxlevel . 20))
 org-enforce-todo-dependencies t
 org-src-tab-acts-natively t
 org-agenda-files '("~/Documents/todo")
 org-reveal-start-hook '(org-decrypt-entry)
 org-blocker-hook '(org-block-todo-from-children-or-siblings-or-parent)
 org-mode-hook '(my/yas-hook org-bullets-mode writegood-mode org-indent-mode flyspell-mode
                 (closure (t) nil (add-hook 'after-save-hook 'my/org-agenda-redo-all nil t))
                 #[0 "\300\301\302\303\304$\207"
                   [add-hook change-major-mode-hook org-show-block-all append local] 5]
                 #[0 "\300\301\302\303\304$\207"
                   [add-hook change-major-mode-hook org-babel-show-result-all append local]
                   5]
                 org-babel-result-hide-spec org-babel-hide-all-hashes
                 org-journal-update-auto-mode-alist org-eldoc-load)
 org-outline-path-complete-in-steps nil
 org-archive-hook '(org-attach-archive-delete-maybe)
 org-journal-carryover-items nil
 org-confirm-elisp-link-function 'yes-or-no-p
 org-journal-mode-hook '(#[nil "\301 \302\303\304$\207"
                           [org-journal-encrypt-on org-add-hook org-journal-encryption-hook
                            nil t]
                           5]
                         (lambda nil (auto-fill-mode t) (flyspell-mode t))
                         (lambda nil
                          (org-add-hook org-journal-encrypt-on 'org-journal-encryption-hook
                           nil t)
                          )
                         )
 org-agenda-before-write-hook '(org-agenda-add-entry-text)
 org-metaup-hook '(org-babel-load-in-session-maybe)
 org-bibtex-headline-format-function #[257 "\300 \236A\207" [:title] 3 "\n\n(fn ENTRY)"]
 org-log-reschedule t
 org-agenda-text-search-extra-files '(agenda-archives)
 org-babel-pre-tangle-hook '(save-buffer)
 org-file-apps '((auto-mode . emacs) ("\\.mm\\'" . default)
                 ("\\.mp \\'" . "/usr/bin/mpv %s")
                 ("\\.x?html?\\'" . "/usr/bin/firefox %s") ("\\.pdf\\'" . default))
 org-tab-first-hook '(org-babel-hide-result-toggle-maybe org-babel-header-arg-expand)
 org-babel-load-languages '((js . t) (shell . t) (python . t) (org . t))
 org-log-done 'time
 org-journal-date-format "[%Y-%m-%d %a]"
 org-hide-emphasis-markers t
 org-ellipsis " ↴"
 org-src-preserve-indentation t
 org-fontify-done-headline t
 org-occur-hook '(org-first-headline-recenter)
 org-agenda-mode-hook '(org-journal-update-org-agenda-files)
 org-structure-template-alist '(("x" "#+BEGIN_EXAMPLE\n?\n#+END_EXAMPLE")
                                ("le" "#+BEGIN_SRC emacs-lisp\n?\n#+END_SRC")
                                ("s" "#+BEGIN_SRC ?\n\n#+END_SRC")
                                ("e" "#+BEGIN_EXAMPLE\n?\n#+END_EXAMPLE")
                                ("q" "#+BEGIN_QUOTE\n?\n#+END_QUOTE")
                                ("v" "#+BEGIN_VERSE\n?\n#+END_VERSE")
                                ("V" "#+BEGIN_VERBATIM\n?\n#+END_VERBATIM")
                                ("c" "#+BEGIN_CENTER\n?\n#+END_CENTER")
                                ("C" "#+BEGIN_COMMENT\n?\n#+END_COMMENT")
                                ("l" "#+BEGIN_EXPORT latex\n?\n#+END_EXPORT")
                                ("L" "#+LaTeX: ")
                                ("h" "#+BEGIN_EXPORT html\n?\n#+END_EXPORT")
                                ("H" "#+HTML: ")
                                ("a" "#+BEGIN_EXPORT ascii\n?\n#+END_EXPORT")
                                ("A" "#+ASCII: ") ("i" "#+INDEX: ?")
                                ("I" "#+INCLUDE: %file ?"))
 org-cycle-hook '(org-cycle-hide-archived-subtrees org-cycle-hide-drawers
                  org-cycle-show-empty-lines org-optimize-window-after-visibility-change)
 org-todo-keywords '((sequence "TODO(t!)" "STARTED(s!)" "NEXT(n!)" "BLOCKED(b@/!)" "|"
                      "DONE(d)")
                     (sequence "IDEA(i)" "|" "CANCELED(c@/!)" "DELEGATED(D@/!)"))
 org-speed-command-hook '(org-speed-command-activate org-babel-speed-command-activate)
 org-journal-dir "~/Documents/journals/daily"
 org-babel-tangle-lang-exts '(("python" . "py") ("emacs-lisp" . "el") ("elisp" . "el"))
 org-return-follows-link t
 org-refile-allow-creating-parent-nodes 'confirm
 org-refile-use-outline-path 'file
 org-confirm-shell-link-function 'yes-or-no-p
 org-link-parameters '(("id" :follow org-id-open)
                       ("rmail" :follow org-rmail-open :store org-rmail-store-link)
                       ("mhe" :follow org-mhe-open :store org-mhe-store-link)
                       ("irc" :follow org-irc-visit :store org-irc-store-link)
                       ("info" :follow org-info-open :export org-info-export :store
                        org-info-store-link) ("gnus" :follow org-gnus-open :store org-gnus-store-link) ("docview" :follow org-docview-open :export org-docview-export :store org-docview-store-link) ("bibtex" :follow org-bibtex-open :store org-bibtex-store-link) ("bbdb" :follow org-bbdb-open :export org-bbdb-export :complete org-bbdb-complete-link :store org-bbdb-store-link) ("w3m" :store org-w3m-store-link) ("file+sys") ("file+emacs")
                       ("doi" :follow org--open-doi-link)
                       ("elisp" :follow org--open-elisp-link)
                       ("file" :complete org-file-complete-link)
                       ("ftp" :follow (lambda (path) (browse-url (concat "ftp:" path))))
                       ("help" :follow org--open-help-link)
                       ("http" :follow (lambda (path) (browse-url (concat "http:" path))))
                       ("https" :follow (lambda (path) (browse-url (concat "https:" path))))
                       ("mailto" :follow
                        (lambda (path) (browse-url (concat "mailto:" path))))
                       ("news" :follow (lambda (path) (browse-url (concat "news:" path))))
                       ("shell" :follow org--open-shell-link))
 org-bullets-bullet-list '("◉" "○" "✸" "✿" "✚" "►" "★")
 org-capture-templates '(("b" "Bookmark" entry
                          (file+headline "/home/n/Documents/todo/bookmarks.org" "Bookmarks")
                          "* [[%x][%^{title}]] %^g\n  :PROPERTIES:\n  :Created: %U\n  :END:")
                         ("l" "Listen")
                         ("la" "Audio Book" entry
                          (file+olp "/home/n/Documents/todo/listen.org" "Listen"
                           "Audio-book")
                          "* TODO %^{Title} %^g %^{Author}p %^{Year}p %^{Genre}p")
                         ("lm" "Music" entry
                          (file+olp "/home/n/Documents/todo/listen.org" "Listen" "Music")
                          "* TODO %^{Title} %^g %^{Artists}p %^{Year}p %^{Genre}p")
                         ("lp" "Podcast" entry
                          (file+olp "/home/n/Documents/todo/listen.org" "Listen" "Podcast")
                          "* TODO %^{Title} %^g \n  :PROPERTIES:\n  :Link(s): [[%^{URL}][%^{Description}]]\n  %^{Genre}\n  :END:")
                         ("lr" "Radio" entry
                          (file+olp "/home/n/Documents/todo/listen.org" "Listen" "Radio")
                          "* TODO %^{Title} %^g \n  :PROPERTIES:\n  :Link(s): [[%^{URL}][%^{Description}]]\n  :Genre: %^{Genre}\n  :END:")
                         ("n" "Note" entry
                          #'(lambda nil
                             (let ((file (concat default-directory "notes.org")))
                              (set-buffer (find-file-noselect file t t)) (write-file file)
                              (goto-char (point-max)) file)
                             )
                          "* TODO %A")
                         ("p" "Play" entry
                          (file+headline "/home/n/Documents/todo/play.org" "Play")
                          "* TODO %^{Title} %^g %^{Year}p %^{Systems}p %^{Info}p")
                         ("r" "Read")
                         ("rf" "fiction" entry
                          (file+olp "/home/n/Documents/todo/read.org" "Read" "Fiction")
                          "%(wikinforg \"book\")")
                         ("rn" "non-fiction" entry
                          (file+olp "/home/n/Documents/todo/read.org" "Read" "Non-Fiction")
                          "%(wikinforg \"book\")")
                         ("s" "Say")
                         ("sw" "word" entry
                          (file+olp "/home/n/Documents/todo/say.org" "Say" "Word")
                          "* %^{Word}\n")
                         ("sp" "Phrase" entry
                          (file+olp "/home/n/Documents/todo/say.org" "Say" "Phrase")
                          "* %^{Phrase}\n")
                         ("sq" "Quote" entry
                          (file+olp "/home/n/Documents/todo/say.org" "Say" "Quote")
                          "* %^{Quote}\n  %^{Quotee}p")
                         ("t" "Todo" entry
                          (file+olp "/home/n/Documents/todo/refile.org" "Refile")
                          "* TODO %^{description} %^g\n%?")
                         ("w" "Watch")
                         ("wf" "Film" entry
                          (file+olp "/home/n/Documents/todo/watch.org" "Watch" "Film")
                          "* TODO %^{Title} %^g %^{Director}p %^{Year}p %^{Genre}p")
                         ("wt" "TV" entry
                          (file+olp "/home/n/Documents/todo/watch.org" "Watch" "TV")
                          "* TODO %^{Title} %^g %^{Year}p %^{Genre}p")
                         ("wp" "Presentation" entry
                          (file+olp "/home/n/Documents/todo/watch.org" "Watch"
                           "Presentation")
                          "* TODO %^{Title} %^g %^{Year}p")
                         )
 org-todo-keyword-faces '(("CANCELED" :foreground "IndianRed1" :weight bold))
 org-journal-time-format "%I:%M %p\njrn"
 org-journal-file-format "%Y.%m.%d.%A.org"
 org-src-window-setup 'current-window
 org-confirm-babel-evaluate nil
 )