Fix erroneous statements about multiply specified JSON columns.
Fix erroneous statements about multiply specified JSON columns.

The behaviour in json_populate_record() and json_populate_recordset()
was changed during development but the docs were not.

jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,xinzweb/gpdb,ovr/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,50wu/gpdb,zeroae/postgres-xl,50wu/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,50wu/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,lisakowen/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,oberstet/postgres-xl,50wu/gpdb,ovr/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,yazun/postgres-xl,oberstet/postgres-xl,lisakowen/gpdb,yazun/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,oberstet/postgres-xl,50wu/gpdb,yazun/postgres-xl,jmcatamney/gpdb,50wu/gpdb,techdragon/Postgres-XL,adam8157/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,ashwinstar/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,pavanvd/postgres-xl,50wu/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,pavanvd/postgres-xl,adam8157/gpdb,ovr/postgres-xl,ovr/postgres-xl,ovr/postgres-xl,yazun/postgres-xl