Document the order of changing certain settings when using hot-standby
servers.  This is just a logical consequence of what was already
documented, but it gives the users some more practical advice.

Author: Yorick Peterse <yorickpeterse@gmail.com>
Reviewed-by: Aleksander Alekseev <a.alekseev@postgrespro.ru>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>