This used to be on the web site but was removed.  The documentation is
a better place for it anyway.

Author: David Rowley <david.rowley@2ndquadrant.com>
Reviewed-by: John Naylor <jcnaylor@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAKJS1f_dKdejdKB94nKZC9S5NzB-UZRcAKkE84e=JEEecDuotg@mail.gmail.com/