[css-align] Clarify that justify-content: stretch flex-end; on a flexbox ignores the 'flex-end' part, since the 'stretch' now behaves as 'flex-start' which doesn't care about fallback alignment.

Author: tabatkins

css-align/Overview.bs

@@ -556,7 +556,8 @@ Content Distribution: the 'justify-content' and 'align-content' properties</h2>
 
 			The 'justify-content' property applies along the <a>main axis</a>,
 			but since flexing in the <a>main axis</a> is controlled by 'flex',
-			''<content-distribution>/stretch'' behaves as ''flex-start''.
+			''<content-distribution>/stretch'' behaves as ''flex-start''
+			(ignoring the specified <a>fallback alignment</a>, if any).
 
 			''justify-content/auto'' behaves as ''<content-distribution>/stretch''.