feat(utxo-lib): clarify MuSig2 variant used in P2TR implementation

Update documentation to clearly differentiate between the older MuSig2 variant
used in BitGo's p2tr script type (chain 30, 31) versus the standard MuSig2
implementation used in p2trMusig2 script type (chain 40, 41).

Issue: BTC-2652

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

modules/utxo-lib/src/taproot.ts

@@ -1,6 +1,7 @@
-// Taproot-specific key aggregation and taptree logic as defined in:
-// https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki
-// https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki
+// This code is used in the BitGo `p2tr` script type (chain 30, 31), which uses
+// an older variant of the MuSig2 key aggregation algorithm.
+// The BitGo `p2trMusig2` script type (chain 40, 41) uses the standard MuSig2
+// key aggregation algorithm which is implemented by the `@brandonblack/musig` library.
 
 import { TapTree as PsbtTapTree, TapLeaf as PsbtTapLeaf } from 'bip174/src/lib/interfaces';
 import assert = require('assert');
@@ -27,10 +28,14 @@ export interface TinySecp256k1Interface {
 }
 
 /**
- * Aggregates a list of public keys into a single MuSig2* public key
- * according to the MuSig2 paper.
+ * Aggregates a list of public keys into a single public key.
+ *
+ * This is an older variant of the MuSig2 key aggregation algorithm that
+ * (1) expects x-only pubkeys with even Y coordinate, and
+ * (2) sorts the keys in ascending order before aggregation.
+ *
  * @param ecc Elliptic curve implementation
- * @param pubkeys The list of pub keys to aggregate
+ * @param pubkeys The list of pub keys to aggregate (32 byte x-only pubkeys)
  * @returns a 32 byte Buffer representing the aggregate key
  */
 export function aggregateMuSigPubkeys(ecc: TinySecp256k1Interface, pubkeys: Buffer[]): Uint8Array {