pg_upgrade docs: mention modification of postgresql.conf in new cluster

Mention it might be necessary to modify postgresql.conf in the new
cluster to match the old cluster.

Backpatch to 9.2.

Suggested by user.

doc/src/sgml/pgupgrade.sgml

@@ -402,7 +402,9 @@ pg_upgrade.exe
 
     <para>
      If you modified <filename>pg_hba.conf</> to use <literal>trust</>,
-     restore its original authentication settings.
+     restore its original authentication settings.  It might also be
+     necessary to adjust other configurations files in the new cluster to
+     match the old cluster, e.g. <filename>postgresql.conf</>.
     </para>
    </step>