Add saml plugin information to auth section

The actual documentation don't show that saml2
can be used a method for authentication.

Change-Id: I89fb8b3e06dbb267e37b996252906b66503f7f1e
Co-Authored-By: Iury Gregory Melo Ferreira <iurygregory@gmail.com>
Closes-Bug: 1488244

doc/common/tables/manual/keystone-saml-module.xml

@@ -0,0 +1,24 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<para xmlns="http://docbook.org/ns/docbook" version="5.0">
+  <!-- This file is NOT auto-generated. -->
+  <table rules="all" xml:id="config_table_keystone_saml-module">
+    <caption>Description of authorization configuration options</caption>
+    <col width="50%"/>
+    <col width="50%"/>
+    <thead>
+      <tr>
+        <th>Configuration option = Default value</th>
+        <th>Description</th>
+      </tr>
+    </thead>
+    <tbody>
+      <tr>
+        <th colspan="2">[auth]</th>
+      </tr>
+      <tr>
+        <td><option>saml2</option> = <replaceable>keystone.auth.plugins.mapped.Mapped</replaceable></td>
+        <td>(StrOpt) The saml2 auth plugin module.</td>
+      </tr>
+    </tbody>
+  </table>
+</para>

doc/config-reference/ch_identityconfigure.xml

@@ -78,6 +78,7 @@ options. For installation prerequisites and step-by-step walkthroughs, see the
     <xi:include href="../common/tables/keystone-policy.xml"/>
     <xi:include href="../common/tables/keystone-revoke.xml"/>
     <xi:include href="../common/tables/keystone-role.xml"/>
+    <xi:include href="../common/tables/manual/keystone-saml-module.xml"/>
     <xi:include href="../common/tables/keystone-saml.xml"/>
     <xi:include href="../common/tables/keystone-security.xml"/>
     <xi:include href="../common/tables/keystone-token.xml"/>