Fix the auto-computation in flex-item-align when the element doesn't have a parent.

--HG--
extra : rebase_source : 7908a822f4d6d6935e06243ac409d21034eb33d7

Author: tabatkins

css3-flexbox/Overview.src.html

@@ -1304,7 +1304,8 @@ <h3 id='flex-align'>
 
 	<p>
 		A value of <dfn id="flex-item-align-auto">''auto''</dfn> for 'flex-item-align' 
-		computes to the value of 'flex-align' on the element's parent.  
+		computes to the value of 'flex-align' on the element's parent,
+		or ''stretch'' if the element has no parent.
 		The alignments are defined as:
 
 	<dl>