Doc: add a short summary of available authentication methods.

The "auth-methods" <sect1> used to include descriptions of all our
authentication methods.  Commit 56811e573 promoted its child <sect2>'s
to <sect1>'s, which has advantages but also created some issues:
* The auth-methods page itself is essentially empty/useless.
* Links that pointed to "auth-methods" as a placeholder for all
auth methods were rendered a bit nonsensical.
* DocBook no longer provides a subsection table-of-contents here,
which formerly was a useful if terse summary of available auth methods.

To improve matters, add a handwritten list of all the auth methods.

Per gripe from Dave Cramer.  Back-patch to v11 where the previous
commit came in.

Discussion: https://postgr.es/m/CADK3HH+xQLhcPgg=kWqfogtXGGZr-JdSo=x=WQC0PkAVyxUWyQ@mail.gmail.com

doc/src/sgml/client-auth.sgml

@@ -911,8 +911,101 @@ omicron         bryanh                  guest1
 
  <sect1 id="auth-methods">
   <title>Authentication Methods</title>
+
+  <para>
+   <productname>PostgreSQL</productname> provides various methods for
+   authenticating users:
+
+   <itemizedlist>
+    <listitem>
+     <para>
+      <link linkend="auth-trust">Trust authentication</link>, which
+      simply trusts that users are who they say they are.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-password">Password authentication</link>, which
+      requires that users send a password.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="gssapi-auth">GSSAPI authentication</link>, which
+      relies on a GSSAPI-compatible security library.  Typically this is
+      used to access an authentication server such as a Kerberos or
+      Microsoft Active Directory server.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="sspi-auth">SSPI authentication</link>, which
+      uses a Windows-specific protocol similar to GSSAPI.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-ident">Ident authentication</link>, which
+      relies on an <quote>Identification Protocol</quote> (RFC 1413)
+      service on the client's machine.  (On local Unix-socket connections,
+      this is treated as peer authentication.)
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-peer">Peer authentication</link>, which
+      relies on operating system facilities to identify the process at the
+      other end of a local connection.  This is not supported for remote
+      connections.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-ldap">LDAP authentication</link>, which
+      relies on an LDAP authentication server.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-radius">RADIUS authentication</link>, which
+      relies on a RADIUS authentication server.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-cert">Certificate authentication</link>, which
+      requires an SSL connection and authenticates users by checking the
+      SSL certificate they send.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-pam">PAM authentication</link>, which
+      relies on a PAM (Pluggable Authentication Modules) library.
+     </para>
+    </listitem>
+    <listitem>
+     <para>
+      <link linkend="auth-bsd">BSD authentication</link>, which
+      relies on the BSD Authentication framework (currently available
+      only on OpenBSD).
+     </para>
+    </listitem>
+   </itemizedlist>
+  </para>
+
+  <para>
+   Peer authentication is usually recommendable for local connections,
+   though trust authentication might be sufficient in some circumstances.
+   Password authentication is the easiest choice for remote connections.
+   All the other options require some kind of external security
+   infrastructure (usually an authentication server or a certificate
+   authority for issuing SSL certificates), or are platform-specific.
+  </para>
+
   <para>
-   The following sections describe the authentication methods in more detail.
+   The following sections describe each of these authentication methods
+   in more detail.
   </para>
  </sect1>