Detail zonal provisioning for GCE PD

Author: mattcary

content/en/docs/concepts/storage/storage-classes.md

@@ -333,7 +333,13 @@ using `allowedTopologies`.
 
 {{< note >}}
 `zone` and `zones` parameters are deprecated and replaced with
-[allowedTopologies](#allowed-topologies)
+[allowedTopologies](#allowed-topologies). When [GCE CSI Migration](
+https://kubernetes.io/docs/concepts/storage/volumes/#gce-csi-migration) is
+enabled, a GCE PD volume can be provisioned in a topology that does not match
+any nodes, but any pod trying to use that volume will fail to schedule. With
+legacy pre-migration GCE PD, in this case an error will be produced
+instead at provisioning time. GCE CSI Migration is enabled by default beginning
+in 1.23.
 {{< /note >}}
 
 ### NFS