message: [client] Rewrite obsolete comment
  Closes:
  Approved by: ankushagarwal
diff: @@ -109,14 +109,17 @@ impl ExtendedPrivKey {
  AuthenticationKey::ed25519(&self.get_public())
  }
  - /// Libra specific sign function that is capable of signing an arbitrary HashValue
  - /// NOTE: In Libra, we do not sign the raw bytes of a transaction, instead we sign the raw
  - /// bytes of the sha3 hash of the raw bytes of a transaction. It is important to note that the
  - /// raw bytes of the sha3 hash will be hashed again as part of the ed25519 signature algorithm.
  - /// In other words: In Libra, the message used for signature and verification is the sha3 hash
  - /// of the transaction. This sha3 hash is then hashed again using SHA512 to arrive at the
  - /// deterministic nonce for the EdDSA.
  - /// TODO: rewrite this comment
  + /// Libra specific sign function that is capable of signing an arbitrary
  + /// Serializable value.
  + ///
  + /// NOTE: In Libra, we do not sign the raw bytes of a transaction, but
  + /// those raw bytes prefixed by a domain separation hash.
  + /// Informally signed_bytes = sha3(domain_separator) || lcs_serialization_bytes
  + ///
  + /// The domain separator hash is derived automatically from a `#[derive(CryptoHasher,
  + /// LCSCryptoHash)]` annotation, or can be declared manually in a process
  + /// described in `libra_crypto::hash`.
  + ///
  pub fn sign<T: CryptoHash + Serialize>(&self, msg: &T) -> Ed25519Signature {
  self.private_key.sign(msg)
  }
  