Update broadcast message notes for Geo

What does this MR do?

• Removes note requiring cache flush on every broadcast message post or removal on Geo secondaries
• Add 10.8 Geo upgrade note requiring one last cache flush for broadcast messages to propagate to secondaries

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

Why was this MR needed?

One last cache flush issue was found during 10.8 staging QA: https://gitlab.slack.com/archives/C9W4C89LY/p1525452655000491

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
• EE specific content should be in the top level /ee folder
• 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?