api-ref: Add response fields and response example for snapshot manage

https://developer.openstack.org/api-ref/block-storage/v3/index.html#manage-an-existing-snapshot
Response fields and response example are missing in the
"Manage an existing snapshot" api ref, this is to add them.

Change-Id: I5a3f397e1a4d7e38d35b707f3c164ef7522aea95

api-ref/source/v3/samples/snapshot-manage-response.json

@@ -0,0 +1,17 @@
+{
+    "snapshot": {
+        "status": "creating",
+        "size": 1,
+        "metadata": {
+            "manage-snap-meta1": "value1",
+            "manage-snap-meta3": "value3",
+            "manage-snap-meta2": "value2"
+        },
+        "name": "tempest-SnapshotManageAdminTest-Managed-Snapshot-1317877679",
+        "volume_id": "1df34919-aba7-4a1b-a614-3b409d71ac03",
+        "created_at": "2018-09-26T03:45:03.893592",
+        "description": "tempest-SnapshotManageAdminTest-Managed-Snapshot-Description-49497381",
+        "id": "b6314a71-9d3d-439a-861d-b790def0d693",
+        "updated_at": null
+    }
+}

api-ref/source/v3/snapshot-manage.inc

@@ -53,6 +53,28 @@ Request Example
    :language: javascript
 
 
+Response
+--------
+
+.. rest_parameters:: parameters.yaml
+
+  - snapshot: snapshot_1
+  - status: status_2
+  - size: size
+  - metadata: metadata_7
+  - name: name_5
+  - volume_id: volume_id
+  - created_at: created_at
+  - description: description_8
+  - id: id
+  - updated_at: updated_at
+
+Response Example
+----------------
+
+.. literalinclude:: ./samples/snapshot-manage-response.json
+   :language: javascript
+
 
 List summary of snapshots available to manage
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~