[css-sizing] Fix the for='' values to be comma-separated.

Author: tabatkins

css-sizing/Overview.bs

@@ -233,7 +233,7 @@ New Keywords for 'width' and 'height'</h3>
 		are represented in the width and height properties by the keywords
 		defined above):
 
-	<dl dfn-type=value dfn-for="width min-width max-width height min-height max-height">
+	<dl dfn-type=value dfn-for="width, min-width, max-width, height, min-height, max-height">
 		<dt><dfn>fill</dfn>
 		<dd>
 			Use the <i>fill-available inline-size</i> or <i>fill-available block-size</i>,
@@ -293,7 +293,7 @@ Containing Floats</h3>
 	</pre>
 
 	<dl>
-		<dt><dfn for="min-width min-height" value>contain-floats</dfn>
+		<dt><dfn for="min-width, min-height" value>contain-floats</dfn>
 		<dd>
 			Equivalent to ''min-width/min-content''
 			except that when applied to the block-size of a block box