[css-grid-2] Clarify that overlap in subgrid/parent line names don't somehow exclude those names from the resolved value on the subgrid. Closes w3c#4362.

Author: tabatkins

css-grid-2/Overview.bs

@@ -485,7 +485,8 @@ Resolved Value of a Track Listing</h3>
 	serialized as the ''subgrid'' keyword
 	followed by a list representing each of its lines
 	as a [=line name set=] of all the line's names
-	excluding those adopted from the parent grid,
+	explicitly defined on the subgrid
+	(not including those adopted from the parent grid),
 	without using the ''repeat()'' notation.
 
 	<div class="example">