doc: Improve description of wal_receiver_status_interval

doc: Improve description of wal_receiver_status_interval

This parameter description was previously confusing, telling that a
value of 0 disabled completely status updates.  This is not true as
there are cases where an update is sent while ignoring this parameter
value.  The documentation is improved to outline the difference of
treatment for scheduled status messages and when these are forced.

Reported-by: Dmitriy Kuzmin
Author: Michael Paquier
Reviewed-by: Euler Taveira
Discussion: https://postgr.es/m/161346024420.3455.1345266601055047937@wrigleys.postgresql.org