doc: Clarify synchronous_standby_names parameter.
The synchronous_standby_names GUC allows specifying num_sync,
the number of synchronous standbys transactions must wait for
replies from. This value must be an integer greater than zero.
This commit updates the documentation to clarify this requirement.
Reported-by: Asphator <asphator@gmail.com>
Discussion: https://postgr.es/m/18663-
b02f75cb919f1b60@postgresql.org