[css-grid-3] Mark issue about item-* moving to another spec

Author: tabatkins

css-grid-3/Overview.bs

@@ -1163,7 +1163,11 @@ Graceful Degradation</h2>
 -->
 
 <h2 id="flow-control">
-Generic Layout Item Flow Controls: the 'item-*' properties</h2>
+Appendix A: Generic Layout Item Flow Controls: the 'item-*' properties</h2>
+
+	Issue: This section is likely to move to another spec,
+	such as [[css-display-4]],
+	since it affects multiple display types.
 
 	Multiple layout modes in CSS place their children
 	as atomic "items" organized into rows and/or colummns in their container,