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

[Stefan Kangas]

Michael Heerdegen <michael_heerdegen@web.de> writes:

>> But I think the text should mention that C-c LETTER is strictly
>> reserved for users and will never be bound by Emacs itself.
>
> Sounds good, from my side.

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.
+
 @node Prefix Keymaps
 @subsection Prefix Keymaps