[api-ref] Update volume metadata key api

The patch did:
1. Add the missing PUT api-ref for volume metadata key.
2. fix the metadata key api's request and response example.
Correct the "metadata" with "meta".

Change-Id: I315682ef86f660eb468991a336d9edae520a4189

api-ref/source/v3/parameters.yaml

@@ -101,6 +101,13 @@ key_2:
   in: path
   required: true
   type: string
+key_3:
+  description: |
+    The metadata key name for the metadata that you
+    want to update.
+  in: path
+  required: true
+  type: string
 project_id_path:
   description: |
     The UUID of the project in a multi-tenancy cloud.
@@ -1122,6 +1129,12 @@ message_level:
   in: body
   required: true
   type: string
+meta:
+  description: |
+    The metadata key and value pair for the volume.
+  in: body
+  required: true
+  type: object
 metadata:
   description: |
     One or more metadata key and value pairs for the

api-ref/source/v3/samples/volume-metadata-show-key-response.json

@@ -0,0 +1,5 @@
+{
+  "meta": {
+    "name": "test"
+  }
+}

api-ref/source/v3/samples/volume-metadata-update-key-request.json

@@ -0,0 +1,5 @@
+{
+  "meta": {
+    "name": "new_name"
+  }
+}

api-ref/source/v3/samples/volume-metadata-update-key-response.json

@@ -0,0 +1,5 @@
+{
+  "meta": {
+    "name": "new_name"
+  }
+}

api-ref/source/v3/volumes-v3-volumes.inc

@@ -623,14 +623,14 @@ Response Parameters
 
 .. rest_parameters:: parameters.yaml
 
-   - metadata: metadata_3
+   - meta: meta
 
 
 
 Response Example
 ----------------
 
-.. literalinclude:: ./samples/volume-metadata-show-response.json
+.. literalinclude:: ./samples/volume-metadata-show-key-response.json
    :language: javascript
 
 
@@ -655,6 +655,45 @@ Request
    - key: key_1
 
 
+Update a volume's metadata for a specific key
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. rest_method::  PUT /v3/{project_id}/volumes/{volume_id}/metadata/{key}
+
+Update metadata for a volume for a specific key.
+
+Normal response codes: 200
+
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+   - project_id: project_id_path
+   - volume_id: volume_id_path
+   - key: key_3
+   - meta: meta
+
+Request Example
+---------------
+
+.. literalinclude:: ./samples/volume-metadata-update-key-request.json
+   :language: javascript
+
+Response Parameters
+-------------------
+
+.. rest_parameters:: parameters.yaml
+
+   - meta: meta
+
+Response Example
+----------------
+
+.. literalinclude:: ./samples/volume-metadata-update-key-response.json
+   :language: javascript
+
 
 Get volumes summary
 ~~~~~~~~~~~~~~~~~~~