math/big: Mention sign inversion in the (*Int).Not documentation

[ainar-g]

I would expect that this:

i = big.NewInt(-2)
log.Println(new(big.Int).Not(i).Text(2))

prints -1, but actually it prints 1. The (*Int).Not documentation simply says:

Not sets z = ^x and returns z.

I think that it's not obvious from this sentence that the sign is inverted.

[ericlagergren]

This matches how Go handles signed integer negation: https://play.golang.org/p/BhiwKrHhVj2

^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
and m = -1 for signed x

If it’s a problem with Not, perhaps the doc fix should apply equally to all bitwise methods.

[ainar-g]

I know that the &^ does that, but it does that because of how signed integers work. big.Int doesn't have a “highest bit”, so the behaviour is rather surprising, at least to me.

[bcmills]

CC @griesemer for math/big