[Doc] add description of "plugin update" to api ref

There is a new method "plugins_update" in file[1],
but still didn't add into doc.

[1]: https://github.com/openstack/sahara/blob/master/sahara/api/v10.py#L216-L221

Change-Id: Ieb8e5cb616b27bb997162229e6f3867f5a0a2c3f

api-ref/source/plugins.inc

@@ -135,3 +135,53 @@ Response Example
 
 
 
+
+Update plugin details
+=====================
+
+.. rest_method::  PATCH /v1.1/{tenant_id}/plugins/{plugin_name}
+
+Updates details for a plugin.
+
+
+Normal response codes:202
+Error response codes:
+
+
+Request
+-------
+
+.. rest_parameters:: parameters.yaml
+
+   - tenant_id: url_tenant_id
+   - plugin_name: plugin_name
+
+
+Request Example
+---------------
+
+.. literalinclude:: samples/plugins/plugin-update-request.json
+   :language: javascript
+
+
+
+Response Parameters
+-------------------
+
+.. rest_parameters:: parameters.yaml
+
+   - title: title
+   - versions: versions
+   - description: description
+   - name: name
+
+
+Response Example
+----------------
+
+.. literalinclude:: samples/plugins/plugin-update-response.json
+   :language: javascript
+
+
+
+

api-ref/source/samples/plugins/plugin-update-request.json

@@ -0,0 +1,7 @@
+{
+    "plugin_labels": {
+        "enabled": {
+            "status": false
+        }
+    }
+}

api-ref/source/samples/plugins/plugin-update-response.json

@@ -0,0 +1,32 @@
+{
+  "plugin": {
+    "plugin_labels": {
+      "hidden": {
+        "status": true,
+        "mutable": true,
+        "description": "Existence of plugin or its version is hidden, but still can be used for cluster creation by CLI and directly by client."
+      },
+      "enabled": {
+        "status": false,
+        "mutable": true,
+        "description": "Plugin or its version is enabled and can be used by user."
+      }
+    },
+    "description": "It's a fake plugin that aimed to work on the CirrOS images. It doesn't install Hadoop. It's needed to be able to test provisioning part of Sahara codebase itself.",
+    "versions": [
+      "0.1"
+    ],
+    "tenant_id": "993f53c1f51845e48e013aeb632358d8",
+    "title": "Fake Plugin",
+    "version_labels": {
+      "0.1": {
+        "enabled": {
+          "status": true,
+          "mutable": true,
+          "description": "Plugin or its version is enabled and can be used by user."
+        }
+      }
+    },
+    "name": "fake"
+  }
+}