[css-borders-4] Clean up 'corner-shape' shorthand (#11687)

No need for /, and can have 1-4 values.

Author: noamr

css-borders-4/Overview.bs

@@ -411,7 +411,7 @@ Corner Shaping: the 'corner-shape' and 'corner-*-shape' properties</h4>
 
 	<pre class="propdef">
 		Name: corner-shape
-		Value: <<corner-shape-value>>{1,2} [ / <<corner-shape-value>>{1,2} ]?
+		Value: <<corner-shape-value>>{1,4}
 		Initial: round
 		Applies to: all elements where 'border-radius' can apply
 		Inherited: no