Clarify `compatibility_version` guidance

- explain in `docs/Formula-Cookbook.md` when to bump
  `compatibility_version` and why it exists so contributors can
  minimise unnecessary `brew upgrade` work and dependent
  `revision` bumps
- point `formula_auditor` failures at the new docs and make the
  messages say the bump must happen in the same PR
- cover the new wording in `formula_auditor_spec` so the audit
  output stays actionable