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

[Tino Calancha]

On Tue, 28 Mar 2017, Andreas Schwab wrote:

> > I don't think is wrong, indeed is consistent with the behaviour
> > in Bug#21454.
> > I was just a bit sorprise that the result is different in
> > interactive calls compared with:
> > M-: (cd "/tmp//") RET
> > M-: (default-directory) RET
> > => "/tmp/"
>
> substitute-in-file-name is a feature of reading the argument
> interactively, before the argument is passed to the command.
>
> Andreas.
I see, then it was my misunderstanding.
Thank you Andreas.