Document that txid_visible_in_snapshot() cannot be used with

subtransaction ids.

doc/src/sgml/func.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.468 2009/01/08 00:44:18 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.469 2009/01/08 14:46:50 momjian Exp $ -->
 
  <chapter id="functions">
   <title>Functions and Operators</title>
@@ -12473,7 +12473,7 @@ SELECT typlen FROM pg_type WHERE oid = pg_typeof(33);
       <row>
        <entry><literal><function>txid_visible_in_snapshot</function>(<parameter>bigint</parameter>, <parameter>txid_snapshot</parameter>)</literal></entry>
        <entry><type>boolean</type></entry>
-       <entry>is transaction ID visible in snapshot?</entry>
+       <entry>is transaction ID visible in snapshot? (do not use with subtransaction ids)</entry>
       </row>
      </tbody>
     </tgroup>