📝 Docs: Clarify test isolation as out of scope for parallel tests

[JoshuaKGoldberg]

Documentation Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Spinning out of #4947: we've resolved that running parallel tests in isolation is out of scope for what Mocha has the design scope or maintenance budget to do. This isn't mentioned in docs, either old or now:

• Current/old: https://mochajs.org/#parallel-tests
• Next/new: https://mocha-docs-next.netlify.app/features/parallel-mode

Additional Info

Docs can be a bit finicky to get right, so expect some back-and-forth with us during review 🙂 .