[css-regions-1] Add explicit reference to css-contain

Author: frivoal

css-regions-1/Overview.bs

@@ -280,7 +280,8 @@ Last region</h3>
 	A <a>CSS region</a> is deemed to be the <dfn lt="last usable region | last usable CSS region">last usable region</dfn>
 	in a <a>region chain</a>
 	if it is the first region in that chain to have <a>layout containment</a>,
-	or the last region in the chain if none of them have <a>layout containment</a>.
+	or the last region in the chain if none of them have <a>layout containment</a>
+	(See [[!CSS-CONTAIN-1]]).
 
 <h3 id="named-flow-section">
 Named flows</h3>