> The module documentation should not contain all historical reasons of every design decision.

Sure.  But, for example, there should be an explanation of why foo+foo.denominator could produce an error for a valid implementation of the Integral abc.  Such interoperability is not assumed by the docs, according to provided examples (https://docs.python.org/3/library/numbers.html#implementing-the-arithmetic-operations).

> One of reasons is that type(self) not always have a constructor with compatible interface.

I hardly can imagine a constructor for the Integral type, which doesn't accept a python integer as an argument.