Clarify what to override when subclassing random.Random

Documentation

In current documentation about the random.Random class, it said that overriding getrandbits() is optional, so it may make people think just overriding random() is sufficient to control the behavior of the class. But in practice methods like randbytes use getrandbits, which is implemented in C to get randomness from MT19937. So it has the following behavior:

import random


class CustomRand(random.Random):
    def random(self):
        print("CustomRand.random()")  # does not get called
        return 0.5


myrand = CustomRand()
myrand.seed(123)  # just for demonstrating that randbytes still use MT19937
r1 = myrand.randbytes(4)

random.seed(123)
r2 = random.randbytes(4)

print(r1 == r2)  # True

Which shows that overriding random() only is not sufficient. From the issue #84466, this seems to be the expected behavior (or not?). So I think this should be documented that overriding getrandbits() is not optional.