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

doc/src/sgml/config.sgml

@@ -4503,15 +4503,16 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
        view.  The standby will report
        the last write-ahead log location it has written, the last position it
        has flushed to disk, and the last position it has applied.
-       This parameter's
-       value is the maximum amount of time between reports.  Updates are
-       sent each time the write or flush positions change, or at least as
-       often as specified by this parameter.  Thus, the apply position may
-       lag slightly behind the true position.
+       This parameter's value is the maximum amount of time between reports.
+       Updates are sent each time the write or flush positions change, or as
+       often as specified by this parameter if set to a non-zero value.
+       There are additional cases where updates are sent while ignoring this
+       parameter; for example, when processing of the existing WAL completes
+       or when <varname>synchronous_commit</varname> is set to
+       <literal>remote_apply</literal>.
+       Thus, the apply position may lag slightly behind the true position.
        If this value is specified without units, it is taken as seconds.
-       The default value is 10 seconds.
-       Setting this parameter to zero disables status updates completely.
-       This parameter can only be set in
+       The default value is 10 seconds. This parameter can only be set in
        the <filename>postgresql.conf</filename> file or on the server
        command line.
       </para>