Update one-off policy documentation

What does this MR do and why?

Updates the documentation on one-off policies with up-to-date information, including re-adding content on one-off policies that was lost in !3513 (merged) .

Expected impact & dry-runs

None. Updating docs only.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review