doc: Update pg_receivexlog note
doc: Update pg_receivexlog note

The old note about how to use pg_receivexlog as an alternative to
archive_command was obsoleted by replication slots.

jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb,adam8157/gpdb