policy: document we intentionally don't lower the dust threshold for …

…Taproot

A "correction" of what seemed to be an overlook was initially proposed in
PR bitcoin#22779. It was deemed unnecessary to further reduce the dust level,
so document the intention.

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

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.