Document Shared Runner Minute Factors
We should document how the system works.
Also, it is important to highlight where and how the accumulation would be adjusted.
This doc covers how to set, but not how minutes are accumulated:
https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html#shared-runners-pipeline-minutes-quota-starter-only.
Kenny to review the current state and be the point person.
Related to: #210570 (closed)