Update some comments to clarify who does what in targetlist creation.
Update some comments to clarify who does what in targetlist creation.

No code changes; just avoid blaming query_planner for things it doesn't
really do.

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