[css-variables] nm, make the tainting more specific. w3c#501.

Author: tabatkins

css-variables/Overview.bs

@@ -473,7 +473,7 @@ Using Cascading Variables: the ''var()'' notation</h2>
 			and the ''var()'' function is being used in:
 
 			* the 'animation' property or one of its longhands,
-			* the 'display' property
+			* the 'display' property, such that after substitution the property would have a value that suppresses the element's box (such as ''display/none'' or ''display/contents'')
 
 			treat the <a>custom property</a> as having its initial value for the rest of this algorithm.