doc: requirepeer is a way to avoid spoofing

We already mentioned unix_socket_directories as an option.

Reported-by: https://www.postgresql.org/message-id/45016837-6cf3-3136-f959-763d06a28076%402ndquadrant.com

Backpatch-through: 9.6

Author: bmomjian

doc/src/sgml/runtime.sgml

@@ -1913,7 +1913,7 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
   </para>
 
   <para>
-   The simplest way to prevent spoofing for <literal>local</>
+   On way to prevent spoofing of <literal>local</>
    connections is to use a Unix domain socket directory (<xref
    linkend="guc-unix-socket-directories">) that has write permission only
    for a trusted local user.  This prevents a malicious user from creating
@@ -1925,6 +1925,13 @@ pg_dumpall -p 5432 | psql -d postgres -p 5433
    <filename>/tmp</> cleanup script to prevent removal of the symbolic link.
   </para>
 
+  <para>
+   Another option for <literal>local</> connections is for clients to use
+   <link linkend="libpq-connect-requirepeer"><literal>requirepeer</></>
+   to specify the required owner of the server process connected to
+   the socket.
+  </para>
+
   <para>
    To prevent spoofing on TCP connections, the best solution is to use
    SSL certificates and make sure that clients check the server's certificate.