[css-grid] Make the third bullet point more explicit.

Author: tabatkins

css-grid/Overview.bs

@@ -3370,7 +3370,8 @@ Grid Container Baselines</h3>
 			Otherwise, the grid container's first/last baseline set
 			is <a lt="synthesize baselines">synthesized</a>
 			from the first/last item's (in <a>grid order</a>) content box,
-			or, failing that, from the grid container's content box.
+			or, if there's no <a>grid item</a>,
+			from the grid container's content box.
 	</ol>
 
 	A <a>grid item</a> <dfn export id="baseline-participation">participates in baseline alignment</dfn> in a particular dimension