[css-containment] Paint containment makes the element a stacking context.

Author: tabatkins

css-containment/Overview.bs

@@ -234,7 +234,8 @@ Paint Containment</h3>
 		the contents of the element is clipped to the element's content box,
 		including both the paint of the descendants and their geometry</span>
 	2. The element must act as a containing block for absolutely positioned and fixed positioned descendants.
-	3. The element must be a <a>formatting context</a>.
+	3. The element must be a <a>stacking context</a>.
+	4. The element must be a <a>formatting context</a>.
 
 	Possible optimizations that can be enabled by <a>paint containment</a> include (but are not limited to):
 
@@ -251,4 +252,5 @@ Paint Containment</h3>
 		so there's something to see as soon as the scroll happens,
 		rather than a frame later.)
 
-
+	3. Because they are guaranteed to be stacking contexts,
+		scrolling elements can be painted into a single GPU layer.