Documentation : Geo: External database guide is not correct

We have a document that describes how to setup GitLab Geo with an external database https://docs.gitlab.com/ee/administration/geo/replication/external_database.html and it's not correct as it misses a bunch of steps from https://docs.gitlab.com/ee/administration/geo/replication/database.html

• gitlab-ctl set-geo-primary-node has to be run as Primary Geo Node should be added
• roles ['geo_primary_role'] has to be set (see https://docs.gitlab.com/omnibus/roles/#gitlab-geo-roles)(?)
• roles ['geo_secondary_role'] has to be set on secondary
• geo_secondary['db_fdw'] = true has to be set (well, it's mentioned in https://docs.gitlab.com/ee/administration/geo/replication/external_database.html#configure-the-tracking-database but I believe it should be reworked somehow)
• gitlab-rake geo:db:migrate won't work as database has to be created first

It's a pretty important issue as basically, you can't setup it without deep knowledge of Geo. Moreover, I believe having a database as a separate instance is a common case for huge enterprises that Geo is aiming to target.

Related Slack discussion https://gitlab.slack.com/messages/C32LCGC1H/convo/C32LCGC1H-1551795677.063700/

/cc @devin

UPDATE Also, this fragment of text is duplicated:

To set up an external replica database, you can either:

Set up streaming replication yourself (for example, in AWS RDS).
Perform the Omnibus configuration manually as follows.