CSI migration of Cinder volume plugin is on by default

Document that OpenStack Cinder CSI migration is enabled by default in
Kubernetes 1.21 and that corresponding CSI driver must be installed.

Author: jsafrane

content/en/docs/concepts/storage/volumes.md

@@ -208,14 +208,16 @@ spec:
 
 #### OpenStack CSI migration
 
-{{< feature-state for_k8s_version="v1.18" state="beta" >}}
-
-The `CSIMigration` feature for Cinder, when enabled, redirects all plugin operations
-from the existing in-tree plugin to the `cinder.csi.openstack.org` Container
-Storage Interface (CSI) Driver. In order to use this feature, the [OpenStack Cinder CSI
-Driver](https://github.com/kubernetes/cloud-provider-openstack/blob/master/docs/cinder-csi-plugin/using-cinder-csi-plugin.md)
-must be installed on the cluster and the `CSIMigration` and `CSIMigrationOpenStack`
-beta features must be enabled.
+{{< feature-state for_k8s_version="v1.21" state="beta" >}}
+
+The `CSIMigration` feature for Cinder is enabled by default in Kubernetes 1.21.
+It redirects all plugin operations from the existing in-tree plugin to the
+`cinder.csi.openstack.org` Container Storage Interface (CSI) Driver.
+[OpenStack Cinder CSI Driver](https://github.com/kubernetes/cloud-provider-openstack/blob/master/docs/cinder-csi-plugin/using-cinder-csi-plugin.md)
+must be installed on the cluster.
+The feature can be disabled by un-setting `CSIMigrationOpenStack` beta feature
+gate. When disabled, in-tree Cinder volume plugin will be responsible for
+all storage related tasks, as it was in the previous Kubernetes releases.
 
 ### configMap