bug#66187: read-file-name unexpected behavior when MUSTMATCH is a function

[Eli Zaretskii]

> Cc: philipk@posteo.net, 66187@debbugs.gnu.org
> Date: Tue, 26 Sep 2023 17:55:37 -0700
> From:  Joseph Turner via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> > Finally, I think the docstring of `read-directory-name' needs to be
> > updated: it fails to mention that MUSTMATCH can be a function (the
> > argument is passed directly to `read-file-name').
> 
> How about the attached patch?

This is okay, except that we do document the other arguments.  So why
not just copy/paste the description of MUSTMATCH from read-file-name
to read-directory-name instead?