bug#2470: 23.0.90; auto-fill-mode, :minor-mode-function

1. Someone on EmacsWiki asked how to tell if a given minor mode is
on.  I said to check the mode variable.  The reply was that
auto-fill-mode has no mode variable. 
 
The code shows this:
 
(put 'auto-fill-function :minor-mode-function 'auto-fill-mode)
;; FIXME: turn into a proper minor mode.
;; Add a global minor mode version of it.
(defun auto-fill-mode (&optional arg)
...
 
So it seems to be in need of a fix, but I see no bug filed against it.
 

2. What is :minor-mode-function?  It is used in a couple of places in
the Emacs Lisp sources, with no comment or other explanation.  It is
not in the Elisp manual.  It seems to be a way to associate a
different function with a minor mode - a function with a different
name from the mode itself.  For example, `auto-fill-function' is the
:minor-mode-function for `auto-fill-mode'.
 
Is this mechanism still needed if `define-minor-mode' is used?  Is it
just for compatibility with XEmacs code and legacy code?
 
How about documenting it - at least with a comment in the code
somewhere: what is for, how and when to use it.
 

In GNU Emacs 23.0.90.1 (i386-mingw-nt5.1.2600)
 of 2009-02-01 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4)'