Merge pull request kubernetes#44295 from ziyi-xie/add-csimigration-in…

…struction

Add information about how existing in-trees PVs and in-tree format yaml file will be handled after CSI migration is completed

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

@@ -1103,6 +1103,23 @@ As a result, operators do not have to make any
 configuration changes to existing Storage Classes, PersistentVolumes or PersistentVolumeClaims
 (referring to in-tree plugins) when transitioning to a CSI driver that supersedes an in-tree plugin.
 
+{{< note >}}
+Existing PVs created by a in-tree volume plugin can still be used in the future without any configuration
+changes, even after the migration to CSI is completed for that volume type, and even after you upgrade to a
+version of Kubernetes that doesn't have compiled-in support for that kind of storage.
+
+As part of that migration, you - or another cluster administrator - **must** have installed and configured
+the appropriate CSI driver for that storage. The core of Kubernetes does not install that software for you.
+
+---
+
+After that migration, you can also define new PVCs and PVs that refer to the legacy, built-in
+storage integrations.
+Provided you have the appropriate CSI driver installed and configured, the PV creation continues
+to work, even for brand new volumes. The actual storage management now happens through
+the CSI driver.
+{{< /note >}}
+
 The operations and features that are supported include:
 provisioning/delete, attach/detach, mount/unmount and resizing of volumes.