[css-round-display] Make polar-distance animatable as an angle (ACTION-710: https://www.w3.org/Style/CSS/Tracker/actions/710)

Author: Jihye Hong

css-round-display/Overview.bs

@@ -311,6 +311,7 @@ The 'polar-distance' property specifies the distance from the center of the pare
     Media: visual
     Inherited: no
     Percentages: relative to radius of circle
+    Animatable: as <angle>
 </pre>
 
 <div class='example'>