Properly document the existance of OLD/NEW trigger pl/pgsql trigger fields.
Properly document the existance of OLD/NEW trigger pl/pgsql trigger
fields.

Backpatch to 9.0 and 9.1.

Report from Pavel Stehule, patch from Josh Kupershmidt

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