[css-grid] Make overflow applied to subgrids optional, per WG resolution.

Author: fantasai

css-grid/Overview.bs

@@ -2377,7 +2377,7 @@ Establishing Subgrids: the ''subgrid'' 'display' value</h3>
 			the 'align-content'/'justify-content' properties on it are also ignored.
 
 		<li>
-			However, 'overflow' does apply,
+			The UA <em>may</em> apply 'overflow' to <a>subgrids</a>
 			so the contents of the subgrid can be scrolled aside.
 			(Note: the act of scrolling does not affect layout.)
 
@@ -4117,12 +4117,20 @@ Acknowledgements</h2>
 Changes</h2>
 
 	<ul>
+		<li>
+			Simplify subgrids so that their placement can happen before sizing.
+		<li>Make ''subgrid'' a 'display' value rather than a 'grid-template-*' value,
+			disallowing single-axis subgridding.
+			(Single-axis subgridding may be added in the future,
+			but with a different syntax.)
+		<li>Remove ability to name lines within a subgrid
+			(though this may be added back in a future level).
 		<li>
 			Internal table cell display types are <a>blockified</a>
 			instead of wrapped in an anonymous table.
 			(Updated to match changes to <a href="http://www.w3.org/TR/css-flexbox-1/">Flexbox</a>.)
 		<li>
-			Added <a href="#item-margins"></a>
+			Add <a href="#item-margins"></a>
 			and undefined whether percentages are calculated
 			against the block or inline axis of grid item’s containing block.
 		<li>