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

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 30 Oct 2020 12:47:40 +0100
> Cc: Michael Heerdegen <michael_heerdegen@web.de>, 15917@debbugs.gnu.org,
>  Paul Smith <psmith@gnu.org>
> 
> What about something like:
> 
> Key sequences that consist of @kbd{C-c} followed by a letter (upper or
> lower case) are reserved for users.  Emacs itself will never bind those
> key sequences, and Emacs extensions should avoid binding them.

That'd be fine with me, but should we perhaps clarify what is a
"letter" in this context?  Are we talking about ASCII letters only, or
are non-ASCII letters also included?