Resolve "`promote-to-primary-node` silently fails reconfiguration due to postgresql service not listening"

What does this MR do?

Update the docs to promote a secondary Geo node to a primary node moving the primary domain's DNS record and external_url changes to the end.

Why was this MR needed?

Changing the external_url at the same time that promoting a Geo secondary node into a primary can cause Gitlab::Geo.current_node to return the nil or the wrong node.

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 #4395 (closed)