[css-grid-2] Remove UA option to ignore 'overflow' on subgrids, per Mats' recommendation.

Author: fantasai

css-grid-2/Overview.bs

@@ -195,10 +195,9 @@ Characteristics of a Subgrid Item</h3>
 			in the subgridded dimension.
 
 		<li>
-			The UA <em>may</em> apply 'overflow' to <a>subgrids</a>
+			The 'overflow' property does apply to <a>subgrids</a>,
 			so that overflowing contents of the subgrid can be scrolled into view.
 			(Note: the act of scrolling does not affect layout.)
-			Overflow must otherwise be treated as ''visible''.
 	</ul>
 
 <h3 id="subgrid-sizing">