Api-ref: Add doc for update backup

Cinder added updating backup support in V3.9.

This patch added related api-ref.

Change-Id: I537cb609240e3bcfa130a116e1912d1b660a21f8
Closes-bug: #1607388

api-ref/source/v3/ext-backups.inc

@@ -248,6 +248,52 @@ Response Parameters
    - links: links
    - name: name
 
+
+Update a backup
+~~~~~~~~~~~~~~~
+
+.. rest_method::  PUT /v3/{project_id}/backups/{backup_id}
+
+Update a Block Storage backup. This API is available since v3.9.
+
+Normal response codes: 202,
+Error response codes: Bad Request(400)
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+   - project_id: project_id_path
+   - backup_id: backup_id
+   - backup: backup
+   - description: description
+   - name: name_optional
+
+
+Request Example
+---------------
+
+.. literalinclude:: ./samples/backup-update-request.json
+   :language: javascript
+
+Response Parameters
+-------------------
+
+.. rest_parameters:: parameters.yaml
+
+   - backup: backup
+   - id: id
+   - links: links
+   - name: name
+
+Response Example
+----------------
+
+.. literalinclude:: ./samples/backup-update-response.json
+   :language: javascript
+
+
 List backups for project
 ~~~~~~~~~~~~~~~~~~~~~~~~
 

api-ref/source/v3/samples/backup-update-request.json

@@ -0,0 +1,6 @@
+{
+  "backup":{
+    "name":"test",
+    "description": "this is a backup"
+  }
+}

api-ref/source/v3/samples/backup-update-response.json

@@ -0,0 +1,16 @@
+{
+  "backup": {
+    "id": "fad41a83-203d-4998-9d3b-444fd5da5aba",
+    "links": [
+      {
+        "href": "http://10.3.150.25:8776/v3/a7090a26bc554d93aa845a4d41808251/backups/fad41a83-203d-4998-9d3b-444fd5da5aba",
+        "rel": "self"
+      },
+      {
+        "href": "http://10.3.150.25:8776/a7090a26bc554d93aa845a4d41808251/backups/fad41a83-203d-4998-9d3b-444fd5da5aba",
+        "rel": "bookmark"
+      }
+    ],
+    "name": "test"
+  }
+}