Document that max_worker_processes must be high enough in standby.

Document that max_worker_processes must be high enough in standby.

The setting values of some parameters including max_worker_processes
must be equal to or higher than the values on the master. However,
previously max_worker_processes was not listed as such parameter
in the document. So this commit adds it to that list.

Back-patch to 9.4 where max_worker_processes was added.