bug#56809: file-name-with-extension: Improve docstring.

[Eli Zaretskii]

> From: Damien Cassou <damien@cassou.me>
> Cc: 56809@debbugs.gnu.org
> Date: Thu, 28 Jul 2022 14:01:56 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> > Well, the "return" part was supposed to prevent such an
> > interpretation.  Moreover, that sentence is just the summary.
> 
> Feel free to merge your patch Eli, it's great. I don't want us to keep
> discussing on such a tiny detail.

I already merged it to the emacs-28 (a.k.a. "release") branch.  It
will be merged to master soon enough.

But if someone can come up with a better wording, mine is not carved
in stone.

Thanks, closing.