Add field total to device list in admin API (#8644)

docs/admin_api/user_admin_api.rst

@@ -375,7 +375,8 @@ A response body like the following is returned:
           "last_seen_ts": 1474491775025,
           "user_id": "<user_id>"
         }
-      ]
+      ],
+      "total": 2
     }
 
 **Parameters**
@@ -400,6 +401,8 @@ The following fields are returned in the JSON response body:
     devices was last seen. (May be a few minutes out of date, for efficiency reasons).
   - ``user_id`` - Owner of  device.
 
+- ``total`` - Total number of user's devices.
+
 Delete multiple devices
 ------------------
 Deletes the given devices for a specific ``user_id``, and invalidates