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

[Visuwesh]

[வியாழன் ஜூலை 28, 2022] Damien Cassou wrote:

>> "Return FILENAME modified to…
>
> Most readers will know that FILENAME is not going to be modified but the
> phrasing is still confusing in my opinion. I would prefer a version
> around the word "concatenate" or similar. Here is another version:
>
>   Concatenate FILENAME without its extension and EXTENSION.

I find this confusing.  Eli's version looks better (the other one which
mentions basename is no good either).