bug#25435: 25.1; auto-fill-mode is enabled with 1, disabled with 0

[Francesco Potortì]

>On Jan 13 2017, Francesco Potortì <pot@gnu.org> wrote:
>> Contrary to the docs, auto-fill-mode called programmatically is enabled
>> with arg 1 and disabled with arg 0.
>
>In which way is that contradicting the docs?  The only case that is
>missing is that 'toggle toggles.

In the help string, I read:

--
With a prefix argument ARG, enable Auto Fill mode if ARG is
positive, and disable it otherwise.  If called from Lisp, enable
the mode if ARG is omitted or nil.
--

I read it as meaning that

(auto-fill-mode)        ; turns the mode on
(auto-fill-mode nil)    ; turns the mode on

while everything else turns it off, but

(auto-fill-mode 'off)   ; does not turn it off

If the behaviour is the intended one, then probably the doc string
should be amended to add the word "Additionally" at the beginning of the
second sentence, like this:

With a prefix argument ARG, enable Auto Fill mode if ARG is positive,
and disable it otherwise.  Additionally, if called from Lisp, enable the
mode if ARG is omitted or nil.