Restrict the wording of volume_mount_group

and specify that all node publish and node stage
rpc calls MUST use same volume_mount_group values.

csi.proto

@@ -1373,12 +1373,11 @@ message NodePublishVolumeRequest {
   // manner, unless otherwise modified by a workload, that they are
   // both readable and writable by said mount group identifier.
   //
-  // If Plugin does not support NodePublishVolume with different
-  // volume_mount_group than the one used during NodeStageVolume
-  // then Plugin MAY return FAILED_PRECONDITION error.
-  // Similarly if SP does not support NodePublishVolume of same volume
-  // on same node but with different volume_mount_group it MAY return
-  // FAILED_PRECONDITION error.
+  // If Plugin has STAGE_UNSTAGE_VOLUME capability then value
+  // specified here MUST be same as volume_mount_group specified
+  // during NodeStageVolume call. When node-publishing same volume
+  // multiple times on same node - all NodePublishVolume calls MUST
+  // specify same value of volume_mount_group.
   // This is an OPTIONAL field.
   string volume_mount_group = 9;
 }
@@ -1612,7 +1611,7 @@ message NodeExpandVolumeRequest {
   // Secrets required by plugin to complete node expand volume request.
   // This field is OPTIONAL. Refer to the `Secrets Requirements`
   // section on how to use this field.
-  map<string, string> secrets = 6 
+  map<string, string> secrets = 6
     [(csi_secret) = true, (alpha_field) = true];
 }