Add service account and token rotation details to docs

What does this MR do?

This MR adds details to the docs to explain the situation encountered here.

What are the relevant issue numbers?

Fix Broken Master Branch in Gemnasium (gitlab-org/gitlab#461650 - closed) • Adam Cohen • 17.2

Does this MR meet the acceptance criteria?

• [-] Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer