Document that txid_visible_in_snapshot() cannot be used with subtransaction ids.
Document that txid_visible_in_snapshot() cannot be used with
subtransaction ids.

adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,50wu/gpdb,adam8157/gpdb,ovr/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,edespino/gpdb,lisakowen/gpdb,tpostgres-projects/tPostgres,yuanzhao/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,yuanzhao/gpdb,jmcatamney/gpdb,lisakowen/gpdb,kmjungersen/PostgresXL,edespino/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,postmind-net/postgres-xl,oberstet/postgres-xl,Postgres-XL/Postgres-XL,edespino/gpdb,Chibin/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,Chibin/gpdb,greenplum-db/gpdb,edespino/gpdb,postmind-net/postgres-xl,ashwinstar/gpdb,postmind-net/postgres-xl,adam8157/gpdb,zeroae/postgres-xl,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,greenplum-db/gpdb,Chibin/gpdb,Chibin/gpdb,snaga/postgres-xl,zeroae/postgres-xl,edespino/gpdb,Postgres-XL/Postgres-XL,Chibin/gpdb,arcivanov/postgres-xl,50wu/gpdb,yazun/postgres-xl,snaga/postgres-xl,Chibin/gpdb,adam8157/gpdb,yuanzhao/gpdb,kmjungersen/PostgresXL,oberstet/postgres-xl,greenplum-db/gpdb,postmind-net/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,yuanzhao/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,yuanzhao/gpdb,edespino/gpdb,edespino/gpdb,jmcatamney/gpdb,Chibin/gpdb,edespino/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,arcivanov/postgres-xl,jmcatamney/gpdb,adam8157/gpdb,arcivanov/postgres-xl,edespino/gpdb,xinzweb/gpdb,yuanzhao/gpdb,tpostgres-projects/tPostgres,kmjungersen/PostgresXL,greenplum-db/gpdb,xinzweb/gpdb,kmjungersen/PostgresXL,techdragon/Postgres-XL,arcivanov/postgres-xl,50wu/gpdb,Chibin/gpdb,oberstet/postgres-xl,Chibin/gpdb,snaga/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL,techdragon/Postgres-XL,postmind-net/postgres-xl,tpostgres-projects/tPostgres,yazun/postgres-xl,xinzweb/gpdb,snaga/postgres-xl,xinzweb/gpdb,arcivanov/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,zeroae/postgres-xl,tpostgres-projects/tPostgres,yuanzhao/gpdb,lisakowen/gpdb,yuanzhao/gpdb,Chibin/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,kmjungersen/PostgresXL,oberstet/postgres-xl,tpostgres-projects/tPostgres,lisakowen/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,snaga/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,arcivanov/postgres-xl,50wu/gpdb,pavanvd/postgres-xl,yazun/postgres-xl,ashwinstar/gpdb,edespino/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,yazun/postgres-xl,yazun/postgres-xl