doc: updated doc for subvolumegroup creation

Signed-off-by: Praveen M <m.praveen@ibm.com>

deploy/csi-config-map-sample.yaml

@@ -20,6 +20,7 @@ kind: ConfigMap
 # NOTE: Make sure you don't add radosNamespace option to a currently in use
 # configuration as it will cause issues.
 # The field "cephFS.subvolumeGroup" is optional and defaults to "csi".
+# NOTE: The given subvolumeGroup must already exist in the filesystem.
 # The "cephFS.netNamespaceFilePath" fields are the various network namespace
 # path for the Ceph cluster identified by the <cluster-id>, This will be used
 # by the CephFS CSI plugin to execute the mount -t in the

docs/deploy-cephfs.md

@@ -241,3 +241,9 @@ However, not all KMS are supported in order to be compatible with
 [fscrypt](https://github.com/google/fscrypt). In general KMS that
 either store secrets to use directly (Vault), or allow access to the
 plain password (Kubernetes Secrets) work.
+
+## CephFS PVC Provisioning
+
+Requires subvolumegroup to be created before provisioning the PVC.
+If the subvolumegroup provided in `ceph-csi-config` ConfigMap is missing
+in the ceph cluster, the PVC creation will fail and will stay in `Pending` state.