doc: Add note about postgres_fdw.application_name.

postgres_fdw.application_name can be any string of any length
and contain even non-ASCII characters.  However when it's passed
to and used as application_name in a foreign server, it's truncated
to less than NAMEDATALEN characters and any characters
other than printable ASCII ones in it will be replaced with question
marks. This commit adds these notes into the docs.

Author: Hayato Kuroda
Reviewed-by: Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/TYCPR01MB5870D1E8B949DAF6D3B84E02F5F29@TYCPR01MB5870.jpnprd01.prod.outlook.com

Author: MasaoFujii

doc/src/sgml/postgres-fdw.sgml

@@ -936,6 +936,16 @@ postgres=# SELECT postgres_fdw_disconnect_all();
       Note that change of this parameter doesn't affect any existing
       connections until they are re-established.
      </para>
+     <para>
+      <varname>postgres_fdw.application_name</varname> can be any string
+      of any length and contain even non-ASCII characters.  However when
+      it's passed to and used as <varname>application_name</varname>
+      in a foreign server, note that it will be truncated to less than
+      <symbol>NAMEDATALEN</symbol> characters and any characters other
+      than printable ASCII ones in it will be replaced with question
+      marks (<literal>?</literal>).
+      See <xref linkend="guc-application-name"/> for details.
+     </para>
     </listitem>
    </varlistentry>
   </variablelist>