doc: Add link from sslinfo to pg_stat_ssl

Reviewed-by: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Discussion: https://www.postgresql.org/message-id/flat/398754d8-6bb5-c5cf-e7b8-22e5f0983caf@2ndquadrant.com/

doc/src/sgml/sslinfo.sgml

@@ -14,6 +14,11 @@
   will return NULL) if the current connection does not use SSL.
  </para>
 
+ <para>
+  Some of the information available through this module can also be obtained
+  using the built-in system view <xref linkend="pg-stat-ssl-view"/>.
+ </para>
+
  <para>
   This extension won't build at all unless the installation was
   configured with <literal>--with-openssl</literal>.