[css-containment] Add issue that was just discussed in Sydney face-to-face meeting (a few minutes ago).

Author: dbaron

css-containment/Overview.bs

@@ -138,6 +138,9 @@ Layout Containment</h3>
 
 	2. The element must be a <a>formatting context</a>.
 
+		Issue: This needs to specify how it becomes a formatting
+		context (and which type it becomes).
+
 	3. If a <a>fragmentation context</a> participates in <a>layout containment</a>,
 		the first element with <a>layout containment</a>
 		affecting the <a>fragmentation context</a>