Document change to use multi-phase init #5563
Description
We should document in the upcoming 0.28 migration guide that PyO3 has switched to multi-phase init. This is unlikely to break users, but it's possible, and we should at least inform them of the technical change so they're aware it happened.
(We could offer an opt-out to use single-phase init for a few releases, but the long-term CPython plan is to deprecate single-phase init so I think it's better if we just move forward with it wholly next release. We can reconsider if users hit massively problematic issues.)