pg_export_snapshot doc

pg_export_snapshot() cannot be used during recovery (i.e. on standby
servers), but it's not documented. IMO this is a bug and should be
fixed. Patch attached.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp