[css-containment] Express paint containment in terms of overflow:clip

Based on this resolution:
https://lists.w3.org/Archives/Public/www-style/2015Apr/0405.html

Author: frivoal

css-containment/Overview.bs

@@ -197,8 +197,12 @@ Paint Containment</h3>
 
 	Giving an element <dfn export>paint containment</dfn> has the following effects:
 
-	1. The contents of the element must be clipped to the element's content box.
-		This includes both the paint of the descendants and their geometry (similar to overflow).
+	1. As specified in [[!CSS-OVERFLOW-3]],
+		the computed value the 'overflow-x' and 'overflow-y' is changed to ''overflow/clip''
+		if it would otherwise have been ''overflow/visible''.
+		<span class=note>This means that regardless of the specified value of 'overflow',
+		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>.
 
@@ -208,7 +212,8 @@ Paint Containment</h3>
 		the UA can directly skip trying to paint its contents,
 		as they're guaranteed to be off-screen/obscured as well.
 
-	2. If the containing element is ''overflow:visible'',
+	2. If the containing element would have been ''overflow:visible'',
+		it becomes ''overflow:clip'' and
 		the UA can reserve "canvas" space for the element exactly the element's size.
 		(In similar, scrollable, situations, like ''overflow: hidden'',
 		it's possible to scroll to the currently-clipped content,