Don’t document declare-function internals

Don’t document declare-function internals

Suggested by Stefan Monnier in:
http://lists.gnu.org/archive/html/emacs-devel/2016-05/msg00618.html
* doc/lispref/functions.texi (Declaring Functions):
* lisp/subr.el (declare-function):
* lisp/emacs-lisp/bytecomp.el:
(byte-compile-macroexpand-declare-function):
Document as (fn file &optional arglist fileonly)
even though it is really (fn file &rest args).