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

[Drew Adams]

> >> +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.
> 
> How about "should not conflict"?  Otherwise, please propose an
> alternative.

Better, yes.  But that could mean that we expect that,
most of the time, they won't (automatically).

Active voice here removes such ambiguity, I think.

  Key sequences that consist of @kbd{C-c} followed by a
  letter are reserved for users.  Please do not write
  code to be used by others that binds these keys.

(Why mention "upper or lower"? Why put "reserved for users"
in quotes?)