[css-grid-3][editorial] Reword caption

Author: tabatkins

css-grid-3/Overview.bs

@@ -1095,8 +1095,13 @@ Alignment and Spacing</h2>
 	<figure>
 		<img src="images/masonry-box.png">
 		<figcaption>
-			The extent of the alignment subject is indicated by the dashed border.
-			(Note that item 1 has a 5px bottom margin here.)
+			The extent of the alignment subject is indicated by the dashed border,
+			while the alignment container is the whole container
+			(indicated by the height of the column dividers).
+			It defaults to start-aligning,
+			but depending on (in this case) 'align-content',
+			the items can move down, as a block,
+			to center- or end-align vertically.
 		</figcaption>
 	</figure>