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