* doc/lispref/display.texi (Image Cache): Document animation cache
argument that clear-image-cache gained in Emacs 29 (bug#56546,
bug#66221).
* lisp/image.el (image-animate-timeout): Note limitation of
clear-image-cache with ImageMagick.
* src/image.c (Fclear_image_cache): Rename animation-cache argument
to animation-filter and expand its description for clarity.
(struct anim_cache, anim_create_cache, image_prune_animation_caches):
Improve commentary.
(mark_image_cache): Replace stale commentary that referred to
forall_images_in_image_cache with description of mark_image_cache.
[HAVE_IMAGEMAGICK] (struct animation_cache): Mention lack of
clear-image-cache support in commentary.
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