[css-grid] Resolve issue about ignoring/triggering on grid-template-*

Author: fantasai

css-grid/Overview.bs

@@ -737,14 +737,12 @@ Establishing Grid Containers: the ''display/grid'', ''inline-grid'', and ''subgr
 			If the element is a <a>grid item</a>
 			(i.e. it is in-flow and its parent is a <a>grid container</a>),
 			this value makes the element a <a>subgrid</a>
-			(which is a special type of <a>grid container</a> box, see [[#subgrids]]).
+			(which is a special type of <a>grid container</a> box, see [[#subgrids]])
+			and consequently ignores its 'grid-template-*' and 'grid-*-gap' properties
+			in favor of adopting the parent grid tracks that it spans.
 
 			Otherwise,
 			it behaves as ''grid''.
-
-			Issue: Should we make this contingent on whether grid-template-* have their initial values,
-			in case we want to expand this behavior in the future?
-			(Since grid-template-* is ignored by subgrids right now.)
 	</dl>
 
 	A <dfn export>grid container</dfn> establishes a new <dfn export>grid formatting context</dfn> for its contents.