Use lexical-binding in button.el

Use lexical-binding in button.el

* lisp/button.el: Use lexical-binding.  Expand Keywords header.
Quote function symbols as such.  Use ;;;-comments where appropriate.
(button): Remove outdated commentary of defface.
(define-button-type, make-button, insert-button, make-text-button)
(insert-text-button): Clarify in docstring that PROPERTIES argument
is a plist.
(button-type-subtype-p, button-has-type-p): Do not overspecify
return value in docstring.
(button-put): Fix typo in commentary.