add PPL security setting documentation (#777)

Signed-off-by: penghuo <penghuo@gmail.com>

docs/user/ppl/admin/security.rst

@@ -0,0 +1,69 @@
+.. highlight:: sh
+
+=================
+Security Settings
+=================
+
+.. rubric:: Table of contents
+
+.. contents::
+   :local:
+   :depth: 1
+
+Introduction
+============
+
+User needs ``cluster:admin/opensearch/ppl`` permission to use PPL plugin. User also needs indices level permission ``indices:admin/mappings/get`` to get field mappings and ``indices:data/read/search*`` to search index.
+
+Using Rest API
+==============
+**--INTRODUCED 2.1--**
+
+Example: Create the ppl_role for test_user. then test_user could use PPL to query ``ppl-security-demo`` index.
+
+1. Create the ppl_role and grand permission to access PPL plugin and access ppl-security-demo index::
+
+    PUT _plugins/_security/api/roles/ppl_role
+    {
+      "cluster_permissions": [
+        "cluster:admin/opensearch/ppl"
+      ],
+      "index_permissions": [{
+        "index_patterns": [
+          "ppl-security-demo"
+        ],
+        "allowed_actions": [
+          "indices:data/read/search*",
+          "indices:admin/mappings/get"
+        ]
+      }]
+    }
+
+2. Mapping the test_user to the ppl_role::
+
+    PUT _plugins/_security/api/rolesmapping/ppl_role
+    {
+      "backend_roles" : [],
+      "hosts" : [],
+      "users" : ["test_user"]
+    }
+
+
+Using Security Dashboard
+========================
+**--INTRODUCED 2.1--**
+
+Example: Create ppl_access permission and add to existing role
+
+1. Create the ppl_access permission::
+
+    PUT _plugins/_security/api/actiongroups/ppl_access
+    {
+      "allowed_actions": [
+        "cluster:admin/opensearch/ppl"
+      ]
+    }
+
+2. Grant the ppl_access permission to ppl_test_role
+
+.. image:: https://user-images.githubusercontent.com/2969395/185448976-6c0aed6b-7540-4b99-92c3-362da8ae3763.png

docs/user/ppl/index.rst

@@ -30,6 +30,8 @@ The query start with search command and then flowing a set of command delimited
 
   - `Plugin Settings <admin/settings.rst>`_
 
+  - `Security Settings <admin/security.rst>`_
+
   - `Monitoring <admin/monitoring.rst>`_
 
 * **Commands**