Document animated image API

Document animated image API

* doc/lispref/display.texi (GIF Images): Mention animation.
Remove commented-out old example of animation.
(Animated Images): New subsection.
* doc/lispref/elisp.texi (Top):
* doc/lispref/vol1.texi (Top):
* doc/lispref/vol2.texi (Top): Add Animated Images menu entry.

* lisp/image-mode.el (image-animate-loop, image-toggle-animation): Doc fixes.

* lisp/image.el (image-animated-p): Doc fix.  Use image-animated-types.
(image-animate-timeout): Doc fix.

* etc/NEWS: Markup.