doc: Update outdated descriptions of wal_status in pg_replication_slots.

doc: Update outdated descriptions of wal_status in pg_replication_slots.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.fujii@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Nisha Moond <nisha.moond412@gmail.com>
Discussion: https://postgr.es/m/78b34e84-2195-4f28-a151-5d204a382fdd@oss.nttdata.com
Backpatch-through: 16