pgsql: Document that txid_visible_in_snapshot() cannot be used with

Log Message:
-----------
Document that txid_visible_in_snapshot() cannot be used with
subtransaction ids.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.468 -> r1.469)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.468&r2=1.469)