It states:

"A function without an in contract means that any values of the function
parameters are allowed. This implies that if any function in an
inheritance hierarchy has no in contract, then in contracts on functions
overriding it have no useful effect."

But my example proves otherwise:

class A
{
    int test(int x)
    {
        return x * 2;
    }
}
    
class B : A
{
    int test(int x)
    in
    {
        assert(x == 20);
    }
    body
    {
        return x * 2;
    }
}

void main() {
    B b = new B;
    b.test(10);
}

This will raise an assert error, which would negate the statement "in contracts on functions overriding it have no useful effect.".