Add v3 api-ref for updating specific extra specs for a volume type

This patch adds v3 api-ref for the following volume type API:
Update specific extra specs assigned to a volume type.

Change-Id: Ib8622870d53cbdc249aa465ce62ed45dea2b4089

api-ref/source/v3/samples/volume-type-specific-extra-specs-update-request.json

@@ -0,0 +1,3 @@
+{
+    "key1": "value1"
+}

api-ref/source/v3/samples/volume-type-specific-extra-specs-update-response.json

@@ -0,0 +1,3 @@
+{
+    "key1": "value1"
+}

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

@@ -172,6 +172,40 @@ Response Example
    :language: javascript
 
 
+Update extra specification for volume type
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. rest_method::  PUT /v3/{project_id}/types/{volume_type_id}/extra_specs/{key}
+
+Update the specific extra specification assigned to a volume type.
+
+
+Normal response codes: 200
+
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+   - project_id: project_id_path
+   - volume_type_id: volume_type_id
+   - key: key_4
+
+Request Example
+---------------
+
+.. literalinclude:: ./samples/volume-type-specific-extra-specs-update-request.json
+   :language: javascript
+
+
+Response Example
+----------------
+
+.. literalinclude:: ./samples/volume-type-specific-extra-specs-update-response.json
+   :language: javascript
+
+
 Delete extra specification for volume type
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~