Update singleton pattern docs in design_patterns.md

Closed Luke "Jared" Bennett requested to merge update-singleton-pattern-docs into master

What does this MR do?

Updates single pattern docs in design_patterns.md as suggest in a recent frontend call.

This change promotes "keep it simple".

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32028