Increase image-dired-show-all-from-dir-max-files to 500

Increase image-dired-show-all-from-dir-max-files to 500

* lisp/image-dired.el (image-dired-show-all-from-dir-max-files):
Increase default to 500 to ask for confirmation much less frequently.
The old value was added before we had asynchronous generation of
thumbnails, when a large number of files would lock up Emacs.  Asking
for confirmation could probably be disabled completely these days, but
let's be conservative and set it to some large number of files for
which some users might want to see a prompt.  It can't hurt.
(image-dired-show-all-from-dir): Never warn if above variable is nil.
(image-dired-bookmark-jump): Let-bind above variable to nil instead of
'most-positive-fixnum'.
* doc/emacs/dired.texi (Image-Dired): Don't mention the above
variable, as it is no longer important enough to deserve the space.
* etc/NEWS: Announce the above change.