[css-overflow] Negative overflow-clip-margin is invalid

Part of w3c#4934

Author: frivoal

css-overflow-3/Overview.bs

@@ -527,6 +527,7 @@ Expanding ''overflow:clip'' Bounds: the 'overflow-clip-margin' property</h3>
 			in exactly the same way as a 'box-shadow' grows
 			due to a positive spread distance
 			(including the effect on rounded corners).
+			Negative values are invalid.
 	</dl>
 
 <h3 id="overflow-propagation">