Document limb bound precondition in backends

As described in #850, `as_bytes` in the u64 backend is only injective when the topmost limb is less than 2^48. Our [Scalar invariant 1](https://github.com/dalek-cryptography/curve25519-dalek/blob/fc23dd4a8660353233d54a8bd1244b920ab5ebdc/curve25519-dalek/src/scalar.rs#L199) should ensure that this precondition is met. We just need to document it.