(define-minor-mode): Warn about use of pre-Emacs-21 style args

(define-minor-mode): Warn about use of pre-Emacs-21 style args

* lisp/emacs-lisp/easy-mmode.el (define-minor-mode):
Use `advertised-calling-convention` to avoid promoting the old
style arguments.  Emit a wanring when old-style arguments are used.
Massage the docstring accordingly.
* doc/lispref/modes.texi (Defining Minor Modes): Document the keyword
arguments rather than the old-style positional arguments.