[css-round-display] Update wording to match <https://lists.w3.org/Archives/Public/www-style/2016Jan/0031>

Author: fantasai

css-round-display/Overview.bs

@@ -759,14 +759,14 @@ Values have the following meanings:
 <dl>
 	<dt><var>polar-angle</var></dt>
 	<dd>
-	Rotate an element by the value of 'polar-angle' property.
+	Computes to the value of 'polar-angle' property.
 	</dd>
 </dl>
 
 <dl>
 	<dt><var>polar-angle-reverse</var></dt>
 	<dd>
-	Rotate an element by the angle of the 'polar-angle' property plus 180 degrees.
+	Computes to the value of the 'polar-angle' property plus 180 degrees.
 	</dd>
 </dl>
 </p>