[css-align] Per WG resolution, baselines of scrollable are clamped to the border box, not the margin box.

Author: tabatkins

css-align/Overview.bs

@@ -1599,8 +1599,8 @@ Determining the Baselines of a Box</h2>
 	must be considered as if scrolled to their origin.
 	Furthermore, if, in the case of a box with non-''visible'' overflow,
 	the resulting position of a first (last) baseline
-	is past a box's end (start) margin edge,
-	its position is clamped to that margin edge.
+	is past a box's end (start) border edge,
+	its position is clamped to that border edge.
 
 	ISSUE: This reflects the latest CSS2.1 errata, however see also discussion of an <a href="https://github.com/w3c/csswg-drafts/issues/766">alternate solution</a> that was previously drafted here.