Resolve "Follow-up from "Docs: Improve to include using sidekiq-cluster via gitlab.rb""

What does this MR do?

Further improves sidekiq-cluster documentation to meet style guidelines

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

Closes #10918 (closed)