[css-overflow-4] Clarify (independent) formatting contexts and ellipsis insertion (#12825)

The sentence as it stood was unclear, but this seems to be the only
reasonable thing it can mean.

Author: frivoal

css-overflow-4/Overview.bs

@@ -729,11 +729,12 @@ Indicating Block-Axis Overflow: the 'block-ellipsis' property</h3>
 		block-ellipsis-repaint-002.html
 	</wpt>
 
-	This property only affects a line box
-	if it immediately precedes either a [=region break=]
-	or a [=clamp point=] in the [=block formatting context=],
-	ignoring any intervening [=absolutely positioned=] element
-	or any element closing boundary.
+	Ignoring any intervening [=absolutely positioned=] element
+	or any element closing boundary,
+	this property only affects a line box
+	if it immediately precedes either a [=region break=] or a [=clamp point=],
+	and the line box is in the same [=independent formatting context=]
+	as the [=clamp point=] or the [=region break=].
 
 	Note: See ''continue: discard'' for a way to generate boxes with such a [=region break=].