api-ref: Add parameter "offset" to list apis

Cinder has supported to list volumes/snapshots/types with
parameter "offset" from v2, so add the missing content.

Change-Id: I939d5131cd1b369653bb03810888b7ad56d46088

api-ref/source/v2/parameters.yaml

@@ -211,6 +211,13 @@ name_volume:
   in: query
   required: false
   type: string
+offset:
+  description: |
+    Used in conjunction with ``limit`` to return a slice of items. ``offset``
+    is where to start in the list.
+  in: query
+  required: false
+  type: integer
 sort:
   description: |
     Comma-separated list of sort keys and optional

api-ref/source/v2/volumes-v2-snapshots.inc

@@ -134,6 +134,7 @@ Request
    - sort_key: sort_key
    - sort_dir: sort_dir
    - limit: limit
+   - offset: offset
    - marker: marker
 
 

api-ref/source/v2/volumes-v2-types.inc

@@ -192,6 +192,7 @@ Request
    - sort_key: sort_key
    - sort_dir: sort_dir
    - limit: limit
+   - offset: offset
    - marker: marker
 
 

api-ref/source/v2/volumes-v2-volumes.inc

@@ -79,6 +79,7 @@ Request
    - tenant_id: tenant_id
    - sort: sort
    - limit: limit
+   - offset: offset
    - marker: marker
 
 
@@ -247,6 +248,7 @@ Request
    - tenant_id: tenant_id
    - sort: sort
    - limit: limit
+   - offset: offset
    - marker: marker