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

[Drew Adams]

> > "Should" has at least two meanings: (1) you should
> > do something (like must but less strong) and (2)
> > something probably will happen.
> >
> > #2 says that we expect that something will likely
> > happen, but it doesn't say who makes it happen.
> > It's passive.  There's no telling someone that
> > you want them to do something.  It's just saying
> > that something will likely happen (somehow).
> >
> > My suggestion is that we not allow such a reading.
> > Remove such ambiguity.
> 
> OK.  How do you propose to do that?
> 
> The full text would be:
> 
>    +Key sequences that consist of @kbd{C-c} followed by a letter (upper or
>    +lower case) are ``reserved for users,'' and should not conflict with
>    +any properly designed Emacs extension or Emacs itself.
> 
> To me, it seems clear that the agent in this case would be either the
> Emacs developers or the package developers.

The active voice can make things clearer here.  Say
that this is by GNU convention.  And say that if code
follows the convention then the keys are free for
personal use.  It's about personal use, not just use
"by users".  Many users provide Elisp code for use
by others.

  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.

What constitutes "a letter" here, I leave up to others.