Documentation on instance Bits Integer should be improved

This instance is pretty special:

• Most bit operations pretend to operate on a "fake 2's complement" form, despite this note in the Integer haddocks:

  Integers are stored in a kind of sign-magnitude form, hence do not expect two's complement form when using bit operations.

• However popCount does not use the fake 2's complement form and instead returns negative results for negative Integers.

• rotate = shift

• The conversion from and to the fake 2's complement form incurs extra overhead for bit operations on negative numbers.

I think it would be good to document this in the haddocks of the Bits instance, and to remove or fix the note in the Integer haddocks.