[css-grid-2] Clarify that implicit named lines (implied via grid-template-areas) are passed through, but the infinite names of implicit grid lines (used to resolve placement against named lines that don't exist) are not. w3c#2564

Author: fantasai

css-grid-2/Overview.bs

@@ -156,6 +156,13 @@ Characteristics of a Subgrid Item</h3>
 			the subgridded lines
 			automatically receive the line names
 			specified on the corresponding lines of the parent <a>grid</a>.
+			This includes any <a lt="implicit named lines">implicit names</a>
+			added by the 'grid-template-areas' property of the parent grid
+			(but does not include the infinite unspecified names
+			assumed on its <a>implicit grid lines</a>
+			to resolve placement against named lines
+			that don't exist in the <a>explicit grid</a>).
+
 			These names are in <em>addition</em> to
 			any line names specified locally on the subgrid.