Merge pull request kubernetes-sigs#2003 from shiftstack/docs_migration_instanceid

📖 Document the change of instanceID in v1beta1

Author: k8s-ci-robot

docs/book/src/topics/crd-changes/v1alpha7-to-v1beta1.md

@@ -13,6 +13,7 @@
       - [Change to serverGroupID](#change-to-servergroupid)
       - [Change to image](#change-to-image)
       - [Removal of imageUUID](#removal-of-imageuuid)
+      - [Change to instanceID](#change-to-instanceid)
       - [Changes to ports](#changes-to-ports)
       - [Addition of floatingIPPoolRef](#addition-of-floatingippoolref)
     - [`OpenStackCluster`](#openstackcluster)
@@ -127,6 +128,10 @@ image:
   id: "72a6a1e6-3e0a-4a8b-9b4c-2d6f9e3e5f0a"
 ```
 
+#### Change to instanceID
+
+In v1beta1, the field `instanceID` moves from `spec` to `status`.
+
 #### Changes to ports
 
 These changes apply to ports specified in both OpenStackMachines and the Bastion.