[css-flexbox-1] Clarify that specified size suggestion can't come fro…

…m an automatic size (to prevent a-r from interfering early).

css-flexbox-1/Overview.bs

@@ -1001,7 +1001,7 @@ Grid doesn't have similarly meaningful shrinkability, so it doesn't need to care
 	<dl>
 		<dt><dfn>specified size suggestion</dfn>
 		<dd>
-			If the item’s [=preferred size|preferred=] [=main size=] is <a>definite</a>,
+			If the item’s [=preferred size|preferred=] [=main size=] is <a>definite</a> and not [=automatic size|automatic=],
 			then the <a>specified size suggestion</a> is that size.
 			It is otherwise undefined.