Update Rails isolation_level docs with new learnings

[trevorturk]

Following up on #220 after encountering unexpected errors when retaining isolation_level = :thread in a high-traffic production application being upgraded from Rails 7.0.8 -> 7.1.3.

My suggestion is to leave the documentation mostly in place, but to clarify that retaining :thread is not recommended after my practical experience. See https://github.com/socketry/falcon/pull/220#issuecomment-1924727682 for a bit more detail including the types of errors I was seeing in production.

Types of Changes

• Documentation

Contribution

• [x] I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

Thanks for this, I'm sure it will help other users!