Reword Language regarding DeleteVolume/DeleteSnapshot

This PR reworded some language regarding DeleteVolume and
DeleteSnapshot.

Author: xing-yang

spec.md

@@ -1015,7 +1015,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 A Controller Plugin MUST implement this RPC call if it has `CREATE_DELETE_VOLUME` capability.
 This RPC will be called by the CO to deprovision a volume.
-If successful, the storage space associated with the volume MUST be released and all the data in the volume SHALL NOT be accessible anymore.
+If successful, the space associated with the volume MAY or MAY NOT be released depending upon the configuration of the back end device including any available retention policies.
 
 This operation MUST be idempotent.
 If a volume corresponding to the specified `volume_id` does not exist or the artifacts associated with the volume do not exist anymore, the Plugin MUST reply `0 OK`.
@@ -1500,7 +1500,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 
 A Controller Plugin MUST implement this RPC call if it has `CREATE_DELETE_SNAPSHOT` capability.
 This RPC will be called by the CO to delete a snapshot.
-If successful, the storage space associated with the snapshot MUST be released and all the data in the snapshot SHALL NOT be accessible anymore.
+If successful, the space associated with the snapshot MAY or MAY NOT be released depending upon the configuration of the back end device including any available retention policies.
 
 This operation MUST be idempotent.
 If a snapshot corresponding to the specified `snapshot_id` does not exist or the artifacts associated with the snapshot do not exist anymore, the Plugin MUST reply `0 OK`.