doc: update doc to delete alpha snapshot

updated upgrade documentation to remove
the snapshot created by alpha driver before
upgrade of CSI driver as beta snapshot is not
backward compatible with the alpha snapshot.

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

docs/ceph-csi-upgrade.md

@@ -2,6 +2,7 @@
 
 - [Ceph-csi Upgrade](#ceph-csi-upgrade)
   - [Pre-upgrade considerations](#pre-upgrade-considerations)
+    - [RBD CSI Snapshot Incompatibility](#rbd-csi-snapshot-incompatibility)
   - [Upgrading from v1.2 to v2.0](#upgrading-from-v12-to-v20)
   - [Upgrading from v2.0 to v2.1](#upgrading-from-v20-to-v21)
     - [Upgrading CephFS](#upgrading-cephfs)
@@ -48,6 +49,58 @@ fuse client or rbd-nbd as of now.
 This guide will walk you through the steps to upgrade the software in a cluster
 from v2.0 to v2.1
 
+### RBD CSI Snapshot Incompatibility
+
+CSI snapshot is moved from Alpha to Beta and is not backward compatible. The snapshots
+created with the Alpha version must be deleted before the upgrade.
+
+Check if we have any `v1alpha1` CRD created in our kubernetes cluster, If there
+is no `v1alpha1` CRD created you can skip this step.
+
+```bash
+[$]kubectl get crd volumesnapshotclasses.snapshot.storage.k8s.io -o yaml |grep v1alpha1
+  - name: v1alpha1
+  - v1alpha1
+[$]kubectl get crd volumesnapshotcontents.snapshot.storage.k8s.io -o yaml |grep v1alpha1
+  - name: v1alpha1
+  - v1alpha1
+[$]kubectl get crd volumesnapshots.snapshot.storage.k8s.io -o yaml |grep v1alpha1
+  - name: v1alpha1
+  - v1alpha1
+```
+
+- List all the volumesnapshots created
+
+```bash
+[$] kubectl get volumesnapshot
+NAME               AGE
+rbd-pvc-snapshot   22s
+```
+
+- Delete all volumesnapshots
+
+```bash
+[$] kubectl delete volumesnapshot rbd-pvc-snapshot
+volumesnapshot.snapshot.storage.k8s.io "rbd-pvc-snapshot" deleted
+```
+
+- List all volumesnapshotclasses created
+
+```bash
+[$] kubectl get volumesnapshotclass
+NAME                      AGE
+csi-rbdplugin-snapclass   86s
+```
+
+- Delete all volumesnapshotclasses
+
+```bash
+[$] kubectl delete volumesnapshotclass csi-rbdplugin-snapclass
+volumesnapshotclass.snapshot.storage.k8s.io "csi-rbdplugin-snapclass" deleted
+```
+
+*Note:* The underlying snapshots on the storage system will be deleted by ceph-csi
+
 ## Upgrading from v1.2 to v2.0
 
 Refer