[css-sizing-3] Clarify that available space as a constraint imposes that constraint on boxes. w3c#4219

Author: fantasai

css-sizing-3/Overview.bs

@@ -139,7 +139,8 @@ Terminology</h2>
 			or an infinite size (when it is <a>indefinite</a>).
 			<a>Available space</a> can alternatively be either a
 			<a>min-content constraint</a> or
-			a <a>max-content constraint</a>.
+			a <a>max-content constraint</a>,
+			which forces boxes laid into it to be laid out under that constraint.
 
 <!--
 			<p>