[css-containment] Add 'content' to the list of scoped properties, because quotes are like counters.

Author: tabatkins

css-containment/Overview.bs

@@ -128,7 +128,7 @@ Strong Containment: the 'contain' property</h2>
 			</ul>
 
 		<li>
-			The 'counter-increment', 'counter-set', 'flow-from', and 'flow-into' properties must be <a lt='scoped property'>scoped</a> to the element's sub-tree.
+			The 'counter-increment', 'counter-set', 'flow-from', 'flow-into', and 'content' (for the purpose of ''open-quote''/etc values) properties must be <a lt='scoped property'>scoped</a> to the element's sub-tree.
 	</ol>
 
 	A <dfn export>scoped property</dfn> has its effects scoped to a particular element or subtree.