[css-align] Clarify where space goes when distributing multi-col columns, and apply distribution in all cases of non-auto 'column-width'. Fixes w3c#1420.

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,
+				with any spacing inserted between column boxes
+				added to the relevant column gaps.
 		<tr>
 			<th>'align-content' Axis
 			<td>
@@ -747,7 +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 both a 'column-count' and 'column-width',
+				In the case of multi-column elements with a non-''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.