Add 'metadata' in backup detail&show API document

Backup metadata is available in backup list&show
APIs since 3.43, update the document.

Change-Id: I056ed13124cdb64b5be8012898fb038e112fbe86

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

@@ -90,6 +90,7 @@ Response Parameters
    - snapshot_id: snapshot_id_2
    - os-backup-project-attr:project_id: os-backup-project-attr:project_id
    - count: count
+   - metadata: metadata_9
 
 Response Example
 ----------------
@@ -143,6 +144,7 @@ Response Parameters
    - data_timestamp: data_timestamp
    - snapshot_id: snapshot_id_2
    - os-backup-project-attr:project_id: os-backup-project-attr:project_id
+   - metadata: metadata_9
 
 Response Example
 ----------------

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

@@ -27,6 +27,9 @@
         "updated_at": "2013-04-02T10:35:27.000000",
         "volume_id": "e5185058-943a-4cb4-96d9-72c184c337d6",
         "is_incremental": true,
-        "has_dependent_backups": false
+        "has_dependent_backups": false,
+        "metadata": {
+            "key": "value"
+        }
     }
 }

api-ref/source/v3/samples/backups-list-detailed-response.json

@@ -25,7 +25,9 @@
             "updated_at": "2013-04-02T10:35:27.000000",
             "volume_id": "e5185058-943a-4cb4-96d9-72c184c337d6",
             "is_incremental": true,
-            "has_dependent_backups": false
+            "has_dependent_backups": false,
+            "metadata": {
+                "key": "value"}
         },
         {
             "availability_zone": "az1",
@@ -52,7 +54,9 @@
             "updated_at": "2013-04-02T10:21:48.000000",
             "volume_id": "e5185058-943a-4cb4-96d9-72c184c337d6",
             "is_incremental": true,
-            "has_dependent_backups": false
+            "has_dependent_backups": false,
+             "metadata": {
+                "key": "value"}
         }
     ],
     "count": 10