[Api-ref] Add api doc for attachment complete

This patch[1][2] introduced an api for attachment complete. It should
be added in correlated api doc.

[1]https://review.openstack.org/#/c/493262/3
[2]https://review.openstack.org/#/c/493263/2

Change-Id: Ic5d310418a8161e1ab3165cf71334f4cda71ee2c
Closes-Bug: #1713728

api-ref/source/v3/attachments.inc

@@ -251,3 +251,29 @@ Response Example
 
 .. literalinclude:: ./samples/attachment-update-response.json
    :language: javascript
+
+
+Complete attachment
+~~~~~~~~~~~~~~~~~~~
+
+.. rest_method::  POST /v3/{project_id}/attachments/{attachment_id}/action
+
+Complete an attachment for a cinder volume.
+
+Normal response codes: 202
+Error response codes: badRequest(400), itemNotFound(404)
+
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+   - project_id: project_id_path
+   - attachment_id: attachment_id_1
+
+Request Example
+---------------
+
+.. literalinclude:: ./samples/attachment-complete.json
+   :language: javascript

api-ref/source/v3/samples/attachment-complete.json

@@ -0,0 +1,4 @@
+{
+    "os-complete": {
+    }
+}