Documentation : Geo: Add necessary tracking DB configuration to HA docs

We learned how to get a standalone tracking DB working in https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6247. See more up-to-date info in the comments below.

After we fix omnibus-gitlab#4134 (closed), we should add any necessary configuration to this doc section: https://docs.gitlab.com/ee/administration/geo/replication/high_availability.html#step-3-configure-the-tracking-database-on-the-secondary-node

The doc doesn't make any sense as-is.