image handling: add cleanup note

Author: MrTango

.gitignore

@@ -9,3 +9,4 @@ pyvenv.cfg
 
 # symlinked from submodule
 docs/volto
+/lib64

docs/classic-ui/images.md

@@ -69,7 +69,7 @@ It can be resolved as follows:
 
 This is useful for caching URLs in Varnish or the browser.
 In case the uploaded image or scale definitions have changed, they will be saved again under a different UID.
-This changes the URL and forces either the browser, or a cache proxy such as Varnish, to fetch it again.
+This changes the URL and forces either the browser, or a cache proxy such as Varnish, to fetch it again. When a scale has changed, the old stored entry in the annotation will be deleted after 24h.
 
 
 (classic-ui-images-image-tag-label)=