[css-values-4] Specify that the percent hint is null in all cases.

Author: tabatkins

css-values-4/Overview.bs

@@ -1784,6 +1784,8 @@ Type Checking</h4>
 			:: The [=math function=] is invalid.
 	       </dl>
 
+	       In all cases, the associated [=percent hint=] is null.
+
 	* The [=CSSNumericValue/type=] of a ''calc()'' expression
 		is the [=CSSNumericValue/type=] of its contained expression.
 		The [=CSSNumericValue/type=] of a ''min()'' or ''max()'' expression