[css-containment] Clarify that geometry is clipped by contain:paint, …

…too, so you don't get scrollbars from invisible abspos or something.

css-containment/Overview.bs

@@ -192,6 +192,7 @@ 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).
 	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>.
 
@@ -203,5 +204,10 @@ Paint Containment</h3>
 
 	2. If the containing element is ''overflow:visible'',
 		the UA can reserve "canvas" space for the element exactly the element's size.
-		(Normally, UAs often predictively overpaint elements somewhat,
-		so scrolls can start showing content immediately.)
+		(In similar, scrollable, situations, like ''overflow: hidden'',
+		it's possible to scroll to the currently-clipped content,
+		so UAs often predictively overpaint somewhat
+		so there's something to see as soon as the scroll happens,
+		rather than a frame later.)
+
+