Improve header comment for LockHasWaitersRelation.
Improve header comment for LockHasWaitersRelation.

Dimitry Ivanov spotted a typo, and I added a bit of wordsmithing.

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