[css-grid-2] Explicitly mention scrollbar gutters in the subgrid size. w3c#6350

Author: tabatkins

css-grid-2/Overview.bs

@@ -3686,7 +3686,7 @@ Subgrids</h2>
 
 			<p id="subgrid-margins">
 			In this process, the sum of the <a>subgrid</a>'s
-			margin, padding, and borders at each edge
+			margin, padding, [=scrollbar gutter=], and border at each edge
 			are applied as an extra layer of (potentially negative) margin
 			to the items at those edges.
 			This extra layer of “margin” accumulates