bug#27982: 25.1.50; expand-file-name docstring on how to traverse the filesystem

[Eli Zaretskii]

> From: Nicolas Richard <youngfrog@members.fsf.org>
> Cc: 27982-done@debbugs.gnu.org
> Date: Wed, 30 Aug 2017 17:43:51 +0200
> 
> Thanks for your help but let me close this.
> 
> I initially thought it was "obviously wrong", but it is not. I think the
> wording is slightly misleading (e.g. "dirname" vs "dir-as-file-name"),
> but nothing important.

I added a note to the doc string to make sure the trailing-slash issue
is not overlooked.

Thanks.