api-ref: Fix parameters for os-volume-attachments.inc

In the following APIs, the 'volume_id' parameters are
'path' parameters. So fix them.

- GET /servers/{server_id}/os-volume_attachments/{volume_id}
- DELETE /servers/{server_id}/os-volume_attachments/{volume_id}

In addition, the descriptions of the parameters are amended.

Change-Id: Iab93acde31639b1471c83b06fb1a77474bb52c48
Closes-Bug: #1770560

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

@@ -117,7 +117,7 @@ Request
 .. rest_parameters:: parameters.yaml
 
   - server_id: server_id_path
-  - volume_id: volume_id
+  - volume_id: volume_id_attached_path
 
 Response
 --------
@@ -189,7 +189,7 @@ Request
 .. rest_parameters:: parameters.yaml
 
   - server_id: server_id_path
-  - volume_id: volume_id
+  - volume_id: volume_id_to_detach_path
 
 Response
 --------