[css-align] Clear up the text a little bit.

Author: tabatkins

css-align/Overview.bs

@@ -1605,7 +1605,9 @@ Baseline Alignment Terminology</h3>
 	This is possible only if they
 
 	<ul>
-		<li>Share an <a>alignment context</a> along an axis perpendicular to their baseline alignment axis.
+		<li>Share an <a>alignment context</a> along an axis perpendicular to the axis they're being baseline-aligned in.
+			(For example, grid items with ''align-self: baseline'' are baseline-aligning along the grid’s block axis,
+			and therefore participate with other items in their row.)
 		<li>Have compatible <a>baseline alignment preferences</a>
 			(i.e., the baselines that want to align are on the same side of the alignment context).
 	</ul>