[css-grid] Clarify that grid-gap does nothing on a subgrid.

Author: tabatkins

css-grid/Overview.bs

@@ -2370,6 +2370,11 @@ Establishing Subgrids: the ''subgrid'' 'display' value</h3>
 				and the <a>grid item's</a> own position.
 			</div>
 
+		<li>
+			The 'grid-gap' properties do not apply to subgrids.
+			(Their items are instead spaced by the 'grid-gap' of their parent grid,
+			since they are laid out in the parent grid.)
+
 		<li>
 			The <a>subgrid</a> does not have any <a>implicit grid tracks</a>;
 			every <a>grid area</a> within it is <a>clamped</a> to its <a>explicit grid</a>.