Add a couple of cross-references to the docs about enum types, per a recent gripe that the info is hard to find.
Add a couple of cross-references to the docs about enum types, per a
recent gripe that the info is hard to find.

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