Clarify content of cloned / restored volumes

When cloning a volume or restoring a snapshot, the resulting volume should be
exact copy of the original volume. This includes any filesystem metadata.
Therefore, when the new volume is larger than the original one, the new
volume may need to resize the filesystem (ie call resize2fs, xfs_growfs, ...) to
make the bigger capacity available to workloads.

spec.md

@@ -725,7 +725,11 @@ Plugins MAY create 3 types of volumes:
 
 - Empty volumes. When plugin supports `CREATE_DELETE_VOLUME` OPTIONAL capability.
 - From an existing snapshot. When plugin supports `CREATE_DELETE_VOLUME` and `CREATE_DELETE_SNAPSHOT` OPTIONAL capabilities.
+  SP SHOULD create a volume that looks like exact copy of the original snapshotted volume at the time the snapshot was taken.
+  If the newly created volume is larger than the original volume and SP supports `EXPAND_VOLUME` node capability, CO SHOULD call `NodeExpandVolume` to make the extra space on the volume available to the workloads. 
 - From an existing volume. When plugin supports cloning, and reports the OPTIONAL capabilities `CREATE_DELETE_VOLUME` and `CLONE_VOLUME`.
+  SP SHOULD create a volume that looks like exact copy of the original volume.
+  If the newly created volume is larger than the original volume and SP supports `EXPAND_VOLUME` node capability, CO SHOULD call `NodeExpandVolume` to make the extra space on the volume available to the workloads. 
 
 ```protobuf
 message CreateVolumeRequest {