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

[Drew Adams]

> None of the above fits, because these are not coding conventions.

Do you prefer "key-binding conventions"?

(elisp) `Key Binding Conventions':

 Don't define 'C-c LETTER' as a key in Lisp programs.  Sequences
 consisting of 'C-c' and a letter (either upper or lower case) are
 reserved for users; they are the *only* sequences reserved for
 users, so do not block them.

 Changing all the Emacs major modes to respect this convention was a
 lot of work; abandoning this convention would make that work go to
 waste, and inconvenience users.  Please comply with it.

> We have a good text proposed already, let's not waste any more time
> trying to make it better.

Clearly I've wasted my time, in response to (more than one)
explicit request for suggested text.  Why am I not surprised?