[css-overflow-3] Resolve overflow clip edge shaping per dbaron's recommendation in #5896

Author: fantasai

css-overflow-3/Overview.bs

@@ -629,12 +629,12 @@ Expanding Clipping Bounds: the 'overflow-clip-margin' property
 	
 
 	The [=overflow clip edge=] is shaped in the corners
-	exactly the same way as a 'box-shadow' with a spread radius
-	of the same cumulative offset from the box’s [=border edge=].
-	See [[css-backgrounds-3#corner-shaping]].
-
-	ISSUE(5896): Should this be applying the 1 + (r-1)3 adjustment
-	to the absolute offset or just to <> edge (applying the offset absolutely)?
+	exactly the same way as an outer box-shadow
+	with a spread radius of the same cumulative offset
+	from the box’s [=border edge=].
+	See [[css-backgrounds-3#corner-shaping]]
+	and [[css-backgrounds-3#shadow-shape]],
+	noting in particular the formula for outsets beyond the [=border edge=].
 
 	Note: This property has no effect on boxes
 	with ''overflow: hidden'' or ''overflow: scroll'',