[css-align] Clarify handling of column-spanning elements. Fixes #1418.

Author: fantasai

css-align/Overview.bs

@@ -726,7 +726,9 @@ The 'justify-content' And 'align-content' Properties</h3>
 		<tr>
 			<th><a lt="alignment subject">Alignment Subject(s)</a>
 			<td>
-				The column boxes.
+				The column boxes
+				(and any column-spanning elements),
+				as a singular unit.
 		<tr>
 			<th>'align-content' Axis
 			<td>