bug#26272: default-directory: Remark that it must be a directory name

[Tino Calancha]

Eli Zaretskii <eliz@gnu.org> writes:

> Here's my suggestion:
>
>   It should be a directory name (as opposed to a directory file-name).
>   On GNU and Unix systems, directory names and in a slash `/'.
>
> This basically says what your original description said, which is spot
> on.
>
> Thanks.
Thank you.  Pushed fix into Emacs-25 with commit:
84938d79698c1725016e2bc6480d5d73cc769a81