Merge bitcoin/bitcoin#22863: policy: document dust threshold for Taproot outputs

d873db7f8f policy: document we intentionally don't lower the dust threshold for Taproot (Antoine Poinsot)

Pull request description:

  Following discussions in #22779 .

ACKs for top commit:
  benthecarman:
    ACK d873db7f8f
  ariard:
    Code Review ACK d873db7
  theStack:
    ACK d873db7f8f

Tree-SHA512: 1f5d20dce767f8a74d57ece47a7f6b881741f508896131b8433600cccf9e4262892603b46521d1bb69d5c83b450f24a16731341072a471c1f2c9adad682af895

src/policy/policy.cpp

@@ -22,7 +22,7 @@ CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
     // so dust is a spendable txout less than
     // 182*dustRelayFee/1000 (in satoshis).
     // 546 satoshis at the default rate of 3000 sat/kvB.
-    // A typical spendable segwit txout is 31 bytes big, and will
+    // A typical spendable segwit P2WPKH txout is 31 bytes big, and will
     // need a CTxIn of at least 67 bytes to spend:
     // so dust is a spendable txout less than
     // 98*dustRelayFee/1000 (in satoshis).
@@ -34,6 +34,11 @@ CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
     int witnessversion = 0;
     std::vector<unsigned char> witnessprogram;
 
+    // Note this computation is for spending a Segwit v0 P2WPKH output (a 33 bytes
+    // public key + an ECDSA signature). For Segwit v1 Taproot outputs the minimum
+    // satisfaction is lower (a single BIP340 signature) but this computation was
+    // kept to not further reduce the dust level.
+    // See discussion in https://github.com/bitcoin/bitcoin/pull/22779 for details.
     if (txout.scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
         // sum the sizes of the parts of a transaction input
         // with 75% segwit discount applied to the script size.