org-clock.el: Adds function form evaluation parameter for the clocktable scope

* lisp/org-clock.el (org-dblock-write:clocktable): Make sure to
  evaluate the function form passed as the scope for the clocktable.

* doc/org.texi: Document the feature of using a function form as the
  scope for the clocktable.

This adds back to the clocktable the capacity to have as scope both a
list of file paths or a function that returns such a list.

Also adds this feature the the manual.
---
 doc/org.texi      | 1 +
 lisp/org-clock.el | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/doc/org.texi b/doc/org.texi
index 6be76d8..fb88827 100644
--- a/doc/org.texi
+++ b/doc/org.texi
@@ -6638,6 +6638,7 @@ be selected:
              tree       @r{the surrounding level 1 tree}
              agenda     @r{all agenda files}
              ("file"..) @r{scan these files}
+             (symbol "argument1" ...) @r{scan the files returned by evaluating the function form}
              file-with-archives    @r{current file and its archives}
              agenda-with-archives  @r{all agenda files, including archives}
 :block       @r{The time block to consider.  This block is specified either}
diff --git a/lisp/org-clock.el b/lisp/org-clock.el
index 65c13fd..3ccd309 100644
--- a/lisp/org-clock.el
+++ b/lisp/org-clock.el
@@ -2370,7 +2370,9 @@ the currently selected interval size."
 		    (`file-with-archives
 		     (and buffer-file-name
 			  (org-add-archive-files (list buffer-file-name))))
-		    ((pred consp) scope)
+                    ((and (pred #'listp) (pred (lambda (scope) (symbolp (car scope)))))
+		     (apply (car scope) (cdr scope)))
+                    ((pred listp) scope)
 		    (_ (or (buffer-file-name) (current-buffer)))))
 	   (block (plist-get params :block))
 	   (ts (plist-get params :tstart))
-- 
TINYCHANGE