[css-values-4] Add <rounding-strategy> to grammar w3c#5915

Author: fantasai

css-values-4/Overview.bs

@@ -4072,6 +4072,7 @@ Syntax</h3>
 	<dfn>&lt;calc-value></dfn> = <<number>> | <<dimension>> | <<percentage>> |
 	               <<calc-constant>> | ( <<calc-sum>> )
 	<dfn>&lt;calc-constant></dfn> = e | pi | infinity | -infinity | NaN
+	<<rounding-strategy>> = nearest | up | down | to-zero
 	</pre>
 
 	In addition, [=whitespace=]