bug#24607: 25.1; comment-dwim doc is confused

[Noam Postavsky <npostavs@users.sourceforge.net>]

"liu.huanjie@yahoo.com" <liu.huanjie@yahoo.com> writes:

>    Today I try to write a function that call comment-dwim, it works
> when I 'M-x comment-dwim', but when I call it as (comment-dwim), it
> says wrong argument number, but after checking the doc and read the
> source, I still can't figure out how to give args, which what I want
> is the current line where my cursor is.
>   I add 'nil' to 'comment-dwim' and fixed, but it will be better to
> add something about what args should a caller give, and that will
> not confused to at least elisp newbie, and it is better.

The docstring says

    (comment-dwim ARG)

    [...]
    Else if a prefix ARG is specified, call ‘comment-kill’.

It's a bit terse, but `(elisp) Prefix Command Arguments' says

   * `nil', meaning there is no prefix argument.

as a possible value for prefix arguments.  I don't think we should
explain that again in every function docstring which takes a prefix
argument.