Add comment about default partition in check_new_partition_bound
Add comment about default partition in check_new_partition_bound

The intention of the test is not immediately obvious, so we need this
much.

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