org-agenda: documentation addition

* lisp/org-agenda.el: Add docstring to `org-agenda-show-log'

lisp/org-agenda.el

@@ -2075,7 +2075,10 @@ works you probably want to add it to `org-agenda-custom-commands' for good."
 (defvar org-agenda-follow-mode nil)
 (defvar org-agenda-entry-text-mode nil)
 (defvar org-agenda-clockreport-mode nil)
-(defvar org-agenda-show-log nil)
+(defvar org-agenda-show-log nil
+  "When non-nil, show the log in the agenda.
+Do not set this directly; instead use
+`org-agenda-start-with-log-mode', which see.")
 (defvar org-agenda-redo-command nil)
 (defvar org-agenda-query-string nil)
 (defvar org-agenda-mode-hook nil