3.13 Better Document `__replace__` on Dataclasses and Named Tuples

[max-muoto]

Documentation

With named tuples and dataclasses now implementing __replace__, it would be good to have some clearer documentation on this somewhere. Type-checkers will likely have to implement support as a specific implementation detail, which MyPy is already starting to do (see here, for example), so having more formal documentation of this fact would helpful.