diff --git a/doc/src/sgml/postgres-fdw.sgml b/doc/src/sgml/postgres-fdw.sgml
index 41c952fbe3..f4210c878f 100644
--- a/doc/src/sgml/postgres-fdw.sgml
+++ b/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>