Update the parameter explain when updating a volume attachment

When we update a volume attachment in nova api, we use volume_id instead
of attachment_id in params in latest api.

Change-Id: I5fc4d0ba3bb1c49dfaba2b2eed056441509cb9da

api-ref/source/os-volume-attachments.inc

@@ -138,13 +138,17 @@ Response
 Update a volume attachment
 ==========================
 
-.. rest_method:: PUT /servers/{server_id}/os-volume_attachments/{attachment_id}
+.. rest_method:: PUT /servers/{server_id}/os-volume_attachments/{volume_id}
 
 Update a volume attachment.
 
 .. note:: This action only valid when the server is in ACTIVE, PAUSED and RESIZED state,
           or a conflict(409) error will be returned.
 
+Policy defaults enable only users with the administrative role or
+the owner of the server to perform this operation. Cloud providers
+can change these permissions through the ``policy.json`` file.
+
 Normal response codes: 202
 
 Error response codes: badRequest(400), unauthorized(401), forbidden(403), itemNotFound(404), conflict(409)
@@ -155,7 +159,7 @@ Request
 .. rest_parameters:: parameters.yaml
 
   - server_id: server_id_path
-  - attachment_id: attachment_id
+  - volume_id: volume_id_swap_src
   - volumeAttachment: volumeAttachment_put
   - volumeId: volumeId_swap