Document what to do to reset Geo tracking database

@abuerer and @mkozono were on a customer call where they attempted to upgraded to Geo 10.4.3 and had to redo database replication. They attempted to move the git-data directory out, but there were many instances of "Unable to move original directory out of the way" (https://gitlab.com/gitlab-org/gitlab-ee/issues/4871).

To workaround this, we:

1. Stopped Sidekiq
2. Renamed the repositories to repositories.old
3. Created a new repositories directory with the same permissions
4. Ran gitlab-rake geo:db:reset
5. Restart Sidekiq

We should document this.

I'm also wondering if the ExclusiveLease for certain projects weren't being cleaned up after stopping Sidekiq?