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+=x...@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e64aa1a39d4b8a9502be8ed8dfd67efd6f6acf28

Modified Files
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doc/src/sgml/monitoring.sgml | 4 ++++
1 file changed, 4 insertions(+)

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