doc:  clarify use of RegisterDynamicBackgroundWorker
doc:  clarify use of RegisterDynamicBackgroundWorker

Document likely use of RegisterDynamicBackgroundWorker by another
background worker.

Reported-by: Chapman Flack

Discussion: https://postgr.es/m/CAB7nPqTdi=J9HH8PPPiEOohebdd+xkgbbhdY7=VbGnZ3CkZXxA@mail.gmail.com

Author: Chapman Flack

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