[css-align] Clarify that only multi-line flex containers are affected. Fixes #1421.

Author: fantasai

css-align/Overview.bs

@@ -749,7 +749,7 @@ The 'justify-content' And 'align-content' Properties</h3>
 				both are defined as described in the column-sizing rules
 				of [[!CSS3-MULTICOL]].
 
-				In the case of multi-column elements with a non-''auto'' 'column-width',
+				In the case of multi-column elements with a non-''column-width/auto'' 'column-width',
 				'justify-content' values other than ''justify-content/normal'' or ''justify-content/stretch''
 				cause the columns to take their specified 'column-width'
 				rather than stretching to fill the container.
@@ -771,6 +771,7 @@ The 'justify-content' And 'align-content' Properties</h3>
 
 				For 'align-content',
 				the <a>flex lines</a>.
+				Note, this only has an effect on <a>multi-line flex containers</a>.
 		<tr>
 			<th>'align-content' Axis
 			<td>