doc: fix reference for finding replication slots to drop

doc: fix reference for finding replication slots to drop

Commit a70bce43fb added instructions on how to recover if PostgreSQL
refuses to issue new transaction IDs because of imminent wraparound,
but when describing how to find replication slots that should be dropped,
it referred to pg_stat_replication where it should have referenced
pg_replication_slots.

In passing, decorate references to views with <structname> tags.

Backpatch to all supported versions.

Reported-By: Sanjaya Waruna <sanjaya.waruna@gmail.com>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/176767268098.1084085.10345048667224193115@wrigleys.postgresql.org
Backpatch-through: 14