[css-sizing-3] Reference WM in terminology section. w3c#5725

Author: tabatkins

css-sizing-3/Overview.bs

@@ -87,6 +87,9 @@ Value Definitions</h3>
 <h2 id="terms">
 Terminology</h2>
 
+	Some key terminology related to coordinate axises and dimensions
+	is defined in [[css-writing-modes-3#abstract-box]].
+
 	<dl export>
 		<dt><dfn>size</dfn>
 		<dd>