f Reference BIP 146/147 and clarify/fix spelling in comment

Author: TheBlueMatt

lightning/src/ln/chan_utils.rs

@@ -502,8 +502,8 @@ pub struct LocalCommitmentTransaction {
 	/// The HTLCs and remote htlc signatures which were included in this commitment transaction.
 	///
 	/// Note that this includes all HTLCs, including ones which were considered dust and not
-	/// acually included in the transaction as it appears on-chain, but who's value factor into fee
-	/// calculations.
+	/// actually included in the transaction as it appears on-chain, but who's value is burned as
+	/// fees and not included in the to_local or to_remote outputs.
 	///
 	/// The remote HTLC signatures in the second element will always be set for non-dust HTLCs, ie
 	/// those for which transaction_output_index.is_some().
@@ -583,7 +583,8 @@ impl LocalCommitmentTransaction {
 
 	pub(crate) fn add_local_sig(&self, funding_redeemscript: &Script, our_sig: Signature) -> Transaction {
 		let mut tx = self.unsigned_tx.clone();
-		tx.input[0].witness.push(Vec::new()); // First is the multisig dummy
+		// First push the multisig dummy, note that due to BIP147 (NULLDUMMY) it must be a zero-length element.
+		tx.input[0].witness.push(Vec::new());
 
 		if self.our_sig_first {
 			tx.input[0].witness.push(our_sig.serialize_der().to_vec());