doc: add dustThreshold explain of P2SH & P2TR

src/policy/policy.cpp

@@ -23,22 +23,42 @@
 #include <cstddef>
 #include <vector>
 
+/**
+ * Calculate the dust threshold for an output based on the given fee rate.
+ *
+ * "Dust" is defined in terms of dustRelayFee, which has units satoshis-per-kilobyte.
+ * If you'd pay more in fees than the value of the output to spend something, then we consider it dust.
+ *
+ * Definitions for different output types:
+ * - A typical spendable non-segwit P2PKH txout is 34 bytes big, and will
+ * need a CTxIn of at least 148 bytes to spend:
+ * 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 non-segwit P2SH txout is 32 bytes big, and will
+ * need a CTxIn of at least 148 bytes to spend:
+ * so dust is a spendable txout less than 180*dustRelayFee/1000 (in satoshis).
+ * 540 satoshis at the default rate of 3000 sat/kvB.
+ * - 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).
+ * 294 satoshis at the default rate of 3000 sat/kvB.
+ * - A typical spendable segwit P2TR txout is 43 bytes big, and will
+ * need a CTxIn of at least 57（used 67, see reason below） bytes to spend:
+ * so dust is a spendable txout less than
+ * 110*dustRelayFee/1000 (in satoshis).
+ * 330 satoshis at the default rate of 3000 sat/kvB.
+ *
+ * The actual dust threshold is calculated by determining the cost to
+ * spend the output at the current fee rate and comparing it to the
+ * output's value.
+ *
+ * @param txout The transaction output to evaluate.
+ * @param dustRelayFeeIn The fee rate used for calculating dust, in satoshis-per-kilobyte.
+ * @return The calculated dust threshold in satoshis.
+ */
 CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFeeIn)
 {
- // "Dust" is defined in terms of dustRelayFee,
- // which has units satoshis-per-kilobyte.
- // If you'd pay more in fees than the value of the output
- // to spend something, then we consider it dust.
- // A typical spendable non-segwit txout is 34 bytes big, and will
- // need a CTxIn of at least 148 bytes to spend:
- // 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 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).
- // 294 satoshis at the default rate of 3000 sat/kvB.
  if (txout.scriptPubKey.IsUnspendable())
  return 0;