[css-values] Clarification per CSSWG ACTION-761 <https://lists.w3.org/Archives/Public/www-style/2016Mar/0356.html> “clarify the thing dbaron wrote above in the spec”

Author: fantasai

css-values-3/Overview.bs

@@ -788,6 +788,9 @@ Percentages: the <<percentage>> type</h3>
 		as in ''calc(10% + 20%)''.
 	</div>
 
+	Note: Specifications should never alternate <<percentage>> in place of a dimension
+	in a grammar unless they are <a>compatible</a>.
+
 <h3 id='dimensions'>
 Numbers with Units: <a>dimensions</a></h3>
 

css-values/Overview.bs

@@ -778,6 +778,9 @@ Percentages: the <<percentage>> type</h3>
 		as in ''calc(10% + 20%)''.
 	</div>
 
+	Note: Specifications should never alternate <<percentage>> in place of a dimension
+	in a grammar unless they are <a>compatible</a>.
+
 <h3 id='dimensions'>
 Numbers with Units: <a>dimensions</a></h3>