pgsql: doc: Update outdated descriptions of wal_status in pg_replicatio

doc: Update outdated descriptions of wal_status in pg_replication_slots.

The documentation for pg_replication_slots previously mentioned only
max_slot_wal_keep_size as a condition under which the wal_status column
could show unreserved or lost. However, since commit be87200,
replication slots can also be invalidated due to horizon or wal_level,
and since commit ac0e33136ab, idle_replication_slot_timeout can also
trigger this state.

This commit updates the description of the wal_status column to
reflect that max_slot_wal_keep_size is not the only cause of the lost state.

Back-patched to v16, where the additional invalidation cases were introduced.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Nisha Moond <nisha(dot)moond412(at)gmail(dot)com>
Discussion: https://postgr.es/m/78b34e84-2195-4f28-a151-5d204a382fdd@oss.nttdata.com
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/db51bdf975e28003f08013a12bc46c0848533edc

Modified Files
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doc/src/sgml/system-views.sgml | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)