Improve definition of the Message struct

[Harshil-Jani]

/// A (hashed) message input to an ECDSA signature.
#[derive(Copy, Clone)]
pub struct Message([u8; constants::MESSAGE_SIZE]);

This is not only used in ECDSA Signature, It is also used into the signing of schnorr signature.

rust-secp256k1/src/schnorr.rs

Line 130 in 7c8270a

pub fn sign_schnorr(&self, msg: &Message, keypair: &KeyPair) -> Signature {

[apoelstra]

Sure, we should say that it's input to an "EC signature" and also document which values are invalid.