Clarify documentation
Clarify documentation

Discussion: https://www.postgresql.org/message-id/flat/20170618071607.GA16418%40nol.local

greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb