Add instructions for scheduled promotion to primary

What does this MR do?

• Emphasizes the problem with allowing continued usage of the primary when promoting a secondary
• Adds a more graceful way to take down the primary (if the option is available to you, i.e. during a planned failover)

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

Why was this MR needed?

From https://gitlab.com/gitlab-org/gitlab-ee/issues/4397#note_55649482

Short-term: Is there a way we can make this a doc-only fix by having admins block all access to GitLab (e.g. through proxy/firewall rules) except allow all secondary traffic?

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?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4397

The long term fix would require https://gitlab.com/gitlab-org/gitlab-ce/issues/19739 to be resolved, and then a Geo documentation update.