[css-flexbox] Specify that min-* stuff works when the main-axis overflow is visible, not both overflows.

--HG--
extra : rebase_source : c83b07cf8d5bb87720fbded7d78fe1429090c4c7

Author: tabatkins

css-flexbox/Overview.bs

@@ -882,7 +882,7 @@ Implied Minimum Size of Flex Items</h3>
 	<dl dfn-type=value dfn-for="min-width min-height">
 		<dt><dfn>auto</dfn>
 		<dd>
-			On a <a>flex item</a> whose 'overflow' is ''overflow/visible'',
+			On a <a>flex item</a> whose 'overflow' is ''overflow/visible'' in the <a>main axis</a>,
 			when specified on the <a>flex item's</a> main-axis min-size property,
 			this keyword specifies as the minimum size the smallest of: