Allow read-multiple-choice to do long-form answers

Allow read-multiple-choice to do long-form answers

* doc/lispref/commands.texi (Reading One Event): Document it.

* lisp/emacs-lisp/rmc.el (read-multiple-choice): Allow using
long-form answers instead of single character ones.
(read-multiple-choice--long-answers): New function.
(read-multiple-choice--short-answers): Refactored out from the
main function.