Document consequences of replacing secrets

[V02460]

Document consequences of replacing secrets. The covered config options are registration_shared_secret, macaroon_secret_key, form_secret and worker_replication_secret.

Even though I looked at the source code to check the added documentation is right, I would appreciate additional verification of the statements made.

In an hand-wavy attempt at classifying how bad the consequences of secret replacement are, I added some explanations as warnings and others as regular paragraphs.

Closes #17971

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)