doc: Add link to login event trigger example

The login event trigger is not listed on the trigger firing matrix
since it's not fired by a command.  Add a link to the example code
page similar to how the other event triggers link to the matrix.

Reported-by: Marcos Pegoraro <marcos@f10.com.br>
Discussion: https://postgr.es/m/CAB-JLwYS+78rX02BZ3wJ9ykVrd2i3O1K+7jzvZKQ0evquyQiLQ@mail.gmail.com

Author: danielgustafsson

doc/src/sgml/event-trigger.sgml

@@ -54,6 +54,11 @@
      the in-progress <literal>login</literal> trigger.
    </para>
 
+   <para>
+     For an example on how to use the <literal>login</literal> event trigger,
+     see <xref linkend="event-trigger-database-login-example"/>.
+   </para>
+
    <para>
      The <literal>ddl_command_start</literal> event occurs just before the
      execution of a <literal>CREATE</literal>, <literal>ALTER</literal>, <literal>DROP</literal>,