doc: mention TM is ignored for to_date/to_timestamp()

Report by Goulven Guillard

doc/src/sgml/func.sgml

@@ -5782,6 +5782,8 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
      <listitem>
       <para>
        <literal>TM</literal> does not include trailing blanks. 
+       <function>to_timestamp</> and <function>to_date</> ignore
+       the <literal>TM</literal> modifier.
       </para>
      </listitem>