SEC-1970: Cleanup of pre authentication documentation

* Removed custom-authentication-provider from documentation
* Rephrased to make the pre authentication documentation a little more concise
* Removed nested () within text (not code)
* Removed user which should have been use

docs/manual/src/docbook/preauth.xml

@@ -162,15 +162,11 @@ class="org.springframework.security.web.authentication.preauth.PreAuthenticatedA
       <security:authentication-provider ref="preauthAuthProvider" />
     </security:authentication-manager>
 ]]>
-</programlisting> We've assumed here that the security namespace is being used for
-          configuration (hence the user of the <literal>custom-filter</literal>,
-            <literal>authentication-manager</literal> and
-            <literal>custom-authentication-provider</literal> elements (you can read more about them
-          in the <link xlink:href="#ns-config">namespace chapter</link>). You would leave these out
-          of a traditional bean configuration. It's also assumed that you have added a
-            <interfacename>UserDetailsService</interfacename> (called
-            <quote>userDetailsService</quote>) to your configuration to load the user's roles.
-        </para>
+</programlisting>  We've assumed here that the <link xlink:href="#ns-config">security namespace</link>
+        is being used for configuration. It's also assumed that you have added a
+        <interfacename>UserDetailsService</interfacename> (called
+        <quote>userDetailsService</quote>) to your configuration to load the user's
+        roles. </para>
       </section>
     </section>
     <section>