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

[Drew Adams <drew.adams@oracle.com>]

> How about the following?
> 
> diff --git a/doc/emacs/custom.texi b/doc/emacs/custom.texi
> index 81874a04aa..d993b652f5 100644
> --- a/doc/emacs/custom.texi
> +++ b/doc/emacs/custom.texi
> @@ -1582,6 +1582,10 @@ Keymaps
>  type properly, it automatically handles such sequences as single input
>  events.
> 
> +Key sequences that consist of @kbd{C-c} followed by a letter (upper or
> +lower case) are ``reserved for users,'' and will never conflict with
> +any properly designed Emacs extension or Emacs itself.

IMO that last part ("will never conflict")
sounds too automatic or passive.

In code conventions we're really _prescribing_
what we recommend that people _do_ or don't do
(active).

Here, we really want to suggest/ask that when
writing code for use by others you do not bind
`C-c' followed by a letter.  It's a _request_
to never produce code that conflicts with this
recommendation.

The point is that these keys are reserved for
users.  Another way to put that is to ask/tell
people writing code to be used by others not
to use these keys in that code.