org.el: document optional argument for org-global-tags-completion-table

lisp/org.el

@@ -12356,7 +12356,9 @@ also TODO lines."
 				   (org-entry-properties pom)))))))
 
 (defun org-global-tags-completion-table (&optional files)
-  "Return the list of all tags in all agenda buffer/files."
+  "Return the list of all tags in all agenda buffer/files.
+Optional FILES argument is a list of files to which can be used
+instead of the agenda files."
   (save-excursion
     (org-uniquify
      (delq nil