diff --git a/doc/src/sgml/postgres-fdw.sgml b/doc/src/sgml/postgres-fdw.sgml
index 41c952fbe3..e71d2b4eea 100644
--- a/doc/src/sgml/postgres-fdw.sgml
+++ b/doc/src/sgml/postgres-fdw.sgml
@@ -933,6 +933,16 @@ postgres=# SELECT postgres_fdw_disconnect_all();
       configuration parameter used when <filename>postgres_fdw</filename>
       establishes a connection to a foreign server.  This overrides
       <varname>application_name</varname> option of the server object.
+     </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>).
+     </para>
       Note that change of this parameter doesn't affect any existing
       connections until they are re-established.
      </para>