Add v3 api-ref for deleting extra spec for a volume type

This patch adds v3 api-ref for the API: delete a specific extra spec
for a volume type.

Change-Id: I9b0c1527247aaebe7dc1acc71a376cb41108ee69

api-ref/source/v3/parameters.yaml

@@ -107,6 +107,12 @@ key_3:
   in: path
   required: true
   type: string
+key_4:
+  description: |
+    The key name for the extra spec that you want to remove.
+  in: path
+  required: true
+  type: string
 message_id:
   description: |
     The UUID of the message.

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

@@ -111,6 +111,27 @@ Response Example
    :language: javascript
 
 
+Delete extra spec for volume type
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. rest_method::  DELETE /v3/{project_id}/types/{volume_type_id}/extra_specs/{key}
+
+Deletes the specific extra spec that assigned to a volume type.
+
+
+Normal response codes: 202
+
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+   - project_id: project_id_path
+   - volume_type_id: volume_type_id
+   - key: key_4
+
+
 Show volume type detail
 ~~~~~~~~~~~~~~~~~~~~~~~