Make eval-defun on a pre-defined defcustom call any :set function

Make eval-defun on a pre-defined defcustom call any :set function

* lisp/emacs-lisp/lisp-mode.el (eval-defun-1): Doc fix.
Respect a defcustom's :set function, if appropriate.
(eval-defun): Doc fix.

* doc/lispref/customize.texi (Variable Definitions): Mention eval-defun
on a defcustom calls the :set function when appropriate.

* etc/NEWS: Mention this.

Fixes: debbugs:109