[css-grid] Re #2261, clarify that the limit includes implicit lines.

Author: tabatkins

css-grid-1/Overview.bs

@@ -927,7 +927,7 @@ Sizing Grid Containers</h3>
 Clamping Overly Large Grids</h3>
 
 	Since memory is limited,
-	UAs may clamp the possible size of the <a>grid</a>
+	UAs may clamp the possible size of the <a>implicit grid</a>
 	to be within a UA-defined limit
 	(which should be at least ten thousand tracks on each side of line 1),
 	dropping all lines outside that limit.