Abbrev suggestions helps users remember to use defined abbrevs

Abbrev suggestions helps users remember to use defined abbrevs

    * lisp/abbrev.el (abbrev-suggest): New defcustom.
    (abbrev-suggest-hint-threshold): New defcustom.
    (abbrev--suggest-get-active-tables-including-parents): New defun.
    (abbrev--suggest-get-active-abbrev-expansions): New defun.
    (abbrev--suggest-count-words): New defun.
    (abbrev--suggest-get-previous-words): New defun.
    (abbrev--suggest-above-threshold): New defun.
    (abbrev--suggest-saved-recommendations): New defvar.
    (abbrev--suggest-inform-user): New defun.
    (abbrev--suggest-shortest-abbrev): New defun.
    (abbrev--suggest-maybe-suggest): New defun.
    (abbrev--suggest-get-totals): New defun.
    (abbrev-suggest-show-report): New defun.
    (expand-abbrev): If the previous word was not an abbrev, maybe
    suggest an abbrev to the user.
    * doc/emacs/abbrevs.texi (Abbrev suggestions): New section.
    * etc/NEWS: Announce abbrev suggestions.