[web-animations-1] Update changelog

Author: birtles

web-animations-1/Overview.bs

@@ -5811,6 +5811,9 @@ The following changes have been made since the <a
     (<a href="https://github.com/w3c/web-animations/issues/201">#201</a>).
 *   Dropped keyframe spacing, distance calculation, and retention of invalid
     keyframe property values.
+*   Moved definitions of animation value procedures to
+    <a href="https://drafts.csswg.org/css-values-4/#combining-values">CSS Values
+    and Units</a>.
 *   Introduced {{Animation/updatePlaybackRate(playbackRate)}} instead which
     maintains the current time and synchronizes with animations running on
     another thread or process.