bug#15917: 24.3.50; manual: mention keys reserved for users

[Eli Zaretskii]

> Date: Fri, 30 Oct 2020 13:15:08 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 15917@debbugs.gnu.org, Paul Smith <psmith@gnu.org>
> 
> >   GNU convention stipulates that key sequences that consist
> >   of C-c followed by a letter are reserved for users.  Emacs
> >   Lisp code that follows the convention does not bind these
> >   keys, so that they are free for personal use.
> 
> Perhaps "GNU coding convention" would be even clearer than
> "GNU convention."

It is wrong to speak about GNU (coding) conventions here, since this
is not about GNU or coding, this is about key bindings in Emacs, and
in Emacs alone.