DOCS: document video-aspect behavior change

Forgotten in commit ae2f8fd0.

DOCS/interface-changes.rst

@@ -20,6 +20,9 @@ Interface changes
 ::
 
  --- mpv 0.10.0 will be released ---
+    - change "video-aspect" property: always settable, even if no video is
+      running; always return the override - if no override is set, return
+      the video's aspect ratio
     - remove disc-nav (DVD, BD) related properties and commands
     - add "option-info/<name>/set-locally" property
     - add --cache-backbuffer; change --cache-default default to 75MB