In end-of-defun, terminate early if no further defun exists

In end-of-defun, terminate early if no further defun exists

Before this change, end-of-defun calls end-of-defun-function even if
point is not necessarily at the beginning of a defun (contrary to what
end-of-defun-function's docstring claims).  Now it terminates early
and doesn't call end-of-defun-function.

* lisp/emacs-lisp/lisp.el (beginning-of-defun-raw): Update docstring
clarifying the return value.
(end-of-defun): Terminate early if beginning-of-defun-raw returns nil.