services api ref: fix field enum value and add missing field

I62072573624c9e579a61affed6a7f8d57a6fdb6d tries to add schema
validation for volume services, and there are some inconsistencies
in the api ref doc.

https://developer.openstack.org/api-ref/block-storage/v3/#list-all-cinder-services
1. As Response Example shows, status takes the value of enabled
   or disabled, not available/unavailable.
2. response may contain backend_state field which is optional.

Change-Id: I2435e3dc7c704de158b479d2ea543409b7686a07

api-ref/source/v3/os-services.inc

@@ -55,6 +55,7 @@ Response Parameters
   - cluster: cluster
   - replication_status: replication_status_1
   - active_backend_id: active_backend_id
+  - backend_state: backend_state
 
 Response Example
 ----------------

api-ref/source/v3/parameters.yaml

@@ -529,6 +529,12 @@ backend_id_target:
   in: body
   required: true
   type: string
+backend_state:
+  description: |
+    The state of storage backend. Only in ``cinder-volume`` service.
+  in: body
+  required: false
+  type: string
 backup:
   description: |
     A ``backup`` object.
@@ -2459,7 +2465,7 @@ service_state_1:
   type: string
 service_status:
   description: |
-    The status of the service. One of ``available`` or ``unavailable``.
+    The status of the service. One of ``enabled`` or ``disabled``.
   in: body
   required: true
   type: string