[css-align] Clarify interaction with flexbox spec

Author: fantasai

css-align/Overview.bs

@@ -55,10 +55,13 @@ Module interactions</h3>
 	model described in [[!CSS21]] chapters 9 and 10 and defines the interaction
 	of these properties with the alignment of table cell content using
 	'vertical-align', as defined in [[!CSS21]] chapter 17.
+
 	The interaction of these properties with
 	Grid Layout [[!CSS3-GRID-LAYOUT]]
 	and Flexible Box Layout [[!CSS3-FLEXBOX]]
 	is defined in their respective modules.
+	The property definitions here supersede those in [[!CSS3-FLEXBOX]]
+	(which have a smaller, earlier subset of permissible values).
 
 	No properties in this module apply to the <code>::first-line</code> or
 	<code>::first-letter</code> pseudo-elements.