Geo: Minor improvements to Disaster Recovery and Planned Failover docs

What does this MR do?

Minor improvements to Disaster Recovery and Planned Failover docs.

• In the Block primary traffic step, add a step to verify the GitLab UI is blocked. E.g. Verify that you cannot visit the primary in browser
• In the Block primary traffic step, add a step to verify pushes are blocked. E.g. Verify that pushing a commit fails over HTTP redundant
• In the Block primary traffic step, add a step to verify pushes are blocked. E.g. Verify that pushing a commit fails over SSH if Git over SSH is enabled
• In the Allow replication to finish as much as possible step, specify exactly which queues are of most interest This is addressed sufficiently in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4920
• Add "FYI remember to remove broadcast message" after failover is complete
• Suggest lowering TTL in the DR doc after preparing for planned failover (this advice is useful for unplanned failovers as well)
• Reassure that changing the external url won’t prevent access via the secondary url, as long as the secondary DNS records are correct.

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

Why was this MR needed?

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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Resolves #4932 (closed)