Clarify the meaning of the matchLabels field of the PVC selector

Signed-off-by: Leonardo Cecchi <[email protected]>

Author: leonardoce

examples/kubernetes/groupsnapshot-v1alpha1.yaml

@@ -7,5 +7,7 @@ spec:
   source:
     selector:
       matchLabels:
+        # The PVCs will need to have this label for it to be
+        # included in the VolumeGroupSnapshot
         app.kubernetes.io/name: postgresql
   volumeGroupSnapshotClassName: csi-hostpath-groupsnapclass