Mitigate a few Geo HA setup pitfalls in the documentation

What does this MR do?

Mitigate a few Geo HA setup pitfalls in the documentation.

• Help avoid "customer had 4 front-end nodes each running a tracking database as well as a dedicated node running a tracking database"
• After making an attempt, consider "breaking out the settings a little more granular to help ensure users have the correct settings on each node"
• "clarify that the gitlab.rb files on the application nodes in each cluster need to be the same". Perhaps a warning?
• Consider "Do we want to use the Omnibus GitLab High Availability Roles as opposed to geo_primary_role['enable'] and geo_secondary_role['enable']?"
• Add a note about setting uid and gid to match across all nodes in the cluster to avoid permission issues.

What are the relevant issue numbers?

Resolves https://gitlab.com/gitlab-org/gitlab-ee/issues/7601

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?