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: omnibus-gitlab#4576 (closed)
Related: gitlab-org/distribution/team-tasks#438 (closed)
Signed-off-by: Robert Marshall rmarshall@gitlab.com