[css-overflow-3] Define that end padding is included in the scrollabl…

…e overflow area except for block containers. Trying again because the last commit was wrong and bad. This time working off https://www.w3.org/TR/2018/WD-css-align-3-20181206/#overflow-scroll-position as a model. #3665

css-overflow-3/Overview.bs

@@ -238,12 +238,14 @@ Scrollable Overflow</h3>
 			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
 
 		<li>
-			Padding on the end-edge sides of the <a>scrollable overflow area</a>
+			Padding on the end-edge sides of the <a>scrollable overflow rectangle</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>.
+			added such that its content can be scrolled to a position
+			that would satisfy the requirements of ''place-content: end'' alignment.
+			Including this padding is optional for <a>block containers</a>
+			in any axis whose corresponding <a>content-distribution property</a>
+			('align-content'/'justify-content')
+			is ''align-content/normal''.
 
 			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>;