Add detail to disaster recovery docs

What does this MR do?

The disaster recovery docs are incomplete and ambiguous. This MR adds more detail.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

From gitlab-org/gitlab-ee#4399:

• Add instructions for taking down primary if it isn’t completely down
• ‘Log-in’ is ambiguous in Log-in to your secondary node with.... Explicitly state SSH
• Specify exactly what gitlab.rb changes are needed to apply to the secondary in order to make it a primary
  • geo_primary_role, geo_secondary_role, listen address, trust_addresses, md5_addresses?
  • Determine if it matters if geo_primary_role['enable'] = false. If it must be true, then we need the trust_auth_cidr_addresses config as well.
  • Decide if we should store a replica of the primary gitlab.rb on the secondary. If so, add instructions during Geo setup (or create an issue to make this happen)
• Once DNS change to primary has propagated, update URL in gitlab.rb to reflect.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/4399