[css-display-3] Mention subgrid's affect on GFC creation. Fixes w3c#6157

Author: tabatkins

css-display-3/Overview.bs

@@ -456,7 +456,10 @@ Inner Display Layout Models: the ''flow'', ''flow-root'', ''table'', ''flex'', '
 		<dt><dfn>grid</dfn>
 		<dd>
 			The element generates a principal <a>grid container</a> box,
-			and establishes a <a>grid formatting context</a>. [[!CSS3-GRID-LAYOUT]]
+			and establishes a <a>grid formatting context</a>. [[!CSS-GRID-1]]
+
+			(Grids using ''subgrid'' might not generate a new [=grid formatting context=];
+			see [[CSS-GRID-2]] for details.)
 
 		<dt><dfn>ruby</dfn>
 		<dd>
@@ -734,7 +737,7 @@ Automatic Box Type Transformations</h3>
 		<ul>
 			<li>Absolute positioning or floating an element <a>blockifies</a> the box’s display type. [[CSS2]]
 			<li>Containment in a <a spec=css-ruby>ruby container</a> <a>inlinifies</a> the box’s display type, as described in [[CSS3RUBY]].
-			<li>A parent with a ''grid'' or ''flex'' 'display' value <a>blockifies</a> the box’s display type. [[CSS3-GRID-LAYOUT]] [[CSS3-FLEXBOX]]
+			<li>A parent with a ''grid'' or ''flex'' 'display' value <a>blockifies</a> the box’s display type. [[CSS-GRID-1]] [[CSS3-FLEXBOX]]
 		</ul>
 	</div>