[css-position-4] Better lt for 'in a higher top layer'

Author: tabatkins

css-position-4/Overview.bs

@@ -159,7 +159,7 @@ or none otherwise
 Two elements are <dfn export>in the same top layer</dfn>
 if they have the same [=top layer root=]
 (including if both are none).
-An element A is <dfn export>in a higher top layer</dfn>
+An element A is <dfn export lt="in a higher top layer|in a lower top layer">in a higher top layer</dfn>
 than an element B
 if A has a [=top layer root=],
 and either B has a [=top layer root=] earlier in the [=Document/top layer=] than A's,