* doc/lispref/modes.texi (Defining Minor Modes): Document it.
* lisp/emacs-lisp/easy-mmode.el (buffer-local-set-state): New macro.
(buffer-local-set-state--get): Helper function.
(buffer-local-restore-state): New function.
* lisp/textmodes/word-wrap-mode.el (word-wrap-whitespace-mode):
Use it to simplify code.