[css-overflow-3] Define that end padding is included in the scrollable overflow area except for block containers (which is where the Web-compat problem is). w3c#3665

Author: fantasai

css-overflow-3/Overview.bs

@@ -238,13 +238,14 @@ Scrollable Overflow</h3>
 			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
 
 		<li>
-			Optionally,
-			additional padding on the end-edge sides,
-			corresponding to the end-side padding of the <a>scroll container</a>, such that the end edges of its in-flow content
-			coincide with the end edges of its content area
-			when scrolled to the end.
-
-			Issue(129): It's not yet clear if including the end-side padding in the scrollable layer is Web-compatible, so this clause is under investigation.
+			Padding on the end-edge sides of the <a>scrollable overflow area</a>
+			representing the end-side padding applied to the <a>scroll container</a>,
+			added such that the end edges of its <a>in-flow</a> content
+			coincide with the end edges of the <a>scroll container</a>’s <a>content box</a>
+			when its contents have been scrolled to the end.
+			Including this padding is optional for <a>block containers</a>.
+
+			Issue(129): It's not yet clear if including the end-side padding in the scrollable layer of block containers is Web-compatible, so this clause is under investigation.
 			It appears that Chrome and Safari include such padding <a href="https://bug748518.bmoattachments.org/attachment.cgi?id=618078">in the block axis</a>;
 			and the behavior in the inline axis is <a href="http://jsbin.com/digadoyohi/1/edit?html,css,output">not</a> <a href="http://jsbin.com/vexahocayi/1/edit?html,css,output">clear</a>.