Tweak Flymake autoloads and dependencies

Tweak Flymake autoloads and dependencies

* lisp/progmodes/elisp-mode.el (emacs-lisp-mode): Add to
flymake-diagnostic-functions here.

* lisp/progmodes/flymake-elisp.el[top]: Don't add to
emacs-lisp-mode-hook.  Don't call flymake-elisp-setup-backends in
every buffer.  (flymake-elisp-checkdoc) (flymake-elisp-byte-compile):
Autoload.  (flymake-elisp-setup-backends): Remove.

* lisp/progmodes/flymake.el: Add some top-level comments.
(flymake-make-diagnostic)
(flymake-mode, flymake-mode-on, flymake-mode-off): Add autoloads

Where to fixup this shit?