[css-round-display] Change polar-distance percentages (ACTION-712: https://www.w3.org/Style/CSS/Tracker/actions/712)

Author: Jihye Hong

css-round-display/Overview.bs

@@ -309,7 +309,7 @@ The 'polar-distance' property specifies the distance from the center of the pare
     Initial: 0
     Media: visual
     Inherited: no
-    Percentages: relative to radius of circle
+    Percentages: relative to distance from center to edge of containing block
     Animatable: as <angle>
 </pre>