Geo: Clarify Geo HA documentation
What does this MR do?
What are the relevant issue numbers?
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 
- 
EE specific content should be in the top level /eefolder
- 
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? 
Closes #6108 (closed)