bug#24897: 24.5; doc for `M' in Dired

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Fri, 11 Oct 2019 16:43:23 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, 24897@debbugs.gnu.org
> 
> +See the man page for \"chmod\" for more information on file mode
> +bits, using the command \\[man] in Emacs.

On GNU/Linux, 'chmod' is documented in the Coreutils Info manual, and
the GNU Coding Standards generally prefer Info to man pages.  So I'd
suggest to point to the Coreutils manual first, and to the man page
only as fallback.

> +This command is generally not relevant on MS-Windows."

This is too extreme.  The correct statement is this:

  Note that on MS-Windows this command can only set or reset
  the `w' (write) bit, but cannot change any other bits.