Document auto-fill-function' in relation to auto-fill-mode'

Fixes: debbugs:2470

lisp/ChangeLog

@@ -2,6 +2,8 @@
 
 	* simple.el (current-kill): Clarify what
 	`interprogram-paste-function' does (bug#7500).
+	(auto-fill-mode): Document `auto-fill-function' in relation to
+	`auto-fill-mode' (bug#2470).
 
 2011-07-16  Lawrence Mitchell  <wence@gmx.li>
 

lisp/simple.el

@@ -5237,14 +5237,16 @@ Some major modes set this.")
 ;; auto-fill-function to nil in a file-local setting is safe and
 ;; can be useful to prevent auto-filling.
 (put 'auto-fill-function 'safe-local-variable 'null)
-;; FIXME: turn into a proper minor mode.
-;; Add a global minor mode version of it.
+
 (define-minor-mode auto-fill-mode
   "Toggle Auto Fill mode.
 With ARG, turn Auto Fill mode on if and only if ARG is positive.
 In Auto Fill mode, inserting a space at a column beyond `current-fill-column'
 automatically breaks the line at a previous space.
 
+When `auto-fill-mode' is on, the `auto-fill-function' variable is
+non-`nil'.
+
 The value of `normal-auto-fill-function' specifies the function to use
 for `auto-fill-function' when turning Auto Fill mode on."
   :variable (eq auto-fill-function normal-auto-fill-function))