Add max_replication_slots to PG HA documentation

- PostgreSQL high availability documentation does not mention
  `max_replication_slots` which must be set to work properly as the
  default value is zero.

Resolves: https://gitlab.com/gitlab-org/gitlab-ce/issues/65700

Related: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/4576

Related: https://gitlab.com/gitlab-org/distribution/team-tasks/issues/438Signed-off-by: Robert Marshall <rmarshall@gitlab.com>

changelogs/unreleased/65700-document-max-replication-slots-pg-ha.yml

+---
+title: Add max_replication_slots to PG HA documentation
+merge_request: 31534
+author:
+type: other

doc/administration/high_availability/database.md

@@ -327,6 +327,7 @@ When installing the GitLab package, do not supply `EXTERNAL_URL` value.
    postgresql['sql_user_password'] = 'POSTGRESQL_PASSWORD_HASH'
    # Replace X with value of number of db nodes + 1
    postgresql['max_wal_senders'] = X
+   postgresql['max_replication_slots'] = X
 
    # Replace XXX.XXX.XXX.XXX/YY with Network Address
    postgresql['trust_auth_cidr_addresses'] = %w(XXX.XXX.XXX.XXX/YY)