Dict watchers: the documented callback contract cannot be satisfied (gh-154710)

Doc/c-api/dict.rst places three requirements on a PyDict_WatchCallback, and they are jointly unsatisfiable. I would like input on which one should yield, because every available fix changes either a documented guarantee or Python-visible behavior, and that is not a call I can make as the reporter. The three, all from the PyDict_WatchCallback entry: Callbacks “may not modify dict or otherwise cause code execution in the callback, as it could modify the dict as a side effect”. “Callbacks occur ...