Add summarization comment about visibility functions.
Add summarization comment about visibility functions.

Add URL about the Halloween problem.

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