* org-agenda.el (org-agenda-compact-blocks): Improve docstring.
(org-agenda-block-separator): Add nil to docstring and customization.
(org-prepare-agenda): Skip agenda block separator additionally if
org-agenda-block-separator is nil.
(org-agenda-overriding-header): Improve docstring.

This change allows to disable the agenda block separator only and also
per block only.  (The variable org-agenda-compact-blocks non-nil
disables agenda block separators together with the line containing the
agenda span name and week number and does this globally for all blocks
and all custom agenda commands.)

Michael