doc: Clarify when backend_xmin in pg_stat_replication can be NULL.

doc: Clarify when backend_xmin in pg_stat_replication can be NULL.

Improve the documentation of pg_stat_replication to explain when
the backend_xmin column becomes NULL. This happens when
a replication slot is used (the xmin is then shown in pg_replication_slots)
or when hot_standby_feedback is disabled.

Author: Renzo Dani <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Discussion: https://postgr.es/m/CA+XOKQAMXzskpdUmj2sg03_5fmiXc2Gs0r3TX1_rmcFcqh+=xQ@mail.gmail.com