docs: update configuration documentation

doc/configuration-file.sgml

@@ -4,6 +4,11 @@
     <secondary>location</secondary>
   </indexterm>
 
+  <indexterm>
+    <primary>configuration</primary>
+    <secondary>repmgr.conf location</secondary>
+  </indexterm>
+
   <title>Configuration file location</title>
   <para>
     <application>repmgr</application> and <application>repmgrd</application>

doc/configuration.sgml

@@ -1,7 +1,24 @@
 <chapter id="configuration" xreflabel="Configuration">
- <title>repmgr configuration</title>
+  <title>repmgr configuration</title>
 
   &configuration-file;
   &configuration-file-settings;
 
+  <sect1 id="configuration-permissions" xreflabel="User permissions">
+    <indexterm>
+      <primary>configuration</primary>
+      <secondary>user permissions</secondary>
+    </indexterm>
+
+    <title>repmgr user permissions</title>
+    <para>
+      &repmgr; will create an extension in the BDR database containing objects
+      for administering &repmgr; metadata. The user defined in the `conninfo`
+      setting must be able to access all objects. Additionally, superuser permissions
+      are required to install the &repmgr; extension. The easiest way to do this
+      is create the &repmgr; user as a superuser, however if this is not
+      desirable, the &repmgr; user can be created as a normal user and a
+      superuser specified with <literal>--superuser</literal> when registering a BDR node.
+    </para>
+  </sect1>
 </chapter>