onchain: Document how the expected witness weight for the close tx

bitcoin/tx.c

@@ -886,12 +886,15 @@ size_t bitcoin_tx_simple_input_weight(bool p2sh)
 
 size_t bitcoin_tx_2of2_input_witness_weight(void)
 {
-	/* witness[0] = ""
-	 * witness[1] = sig
-	 * witness[2] = sig
-	 * witness[3] = 2 key key 2 CHECKMULTISIG
-	 */
-	return 1 + (1 + 0) + (1 + 72) + (1 + 72) + (1 + 1 + 33 + 33 + 1 + 1);
+	return 1 + /* Prefix: 4 elements to push on stack */
+		(1 + 0) + /* [0]: witness-marker-and-flag */
+		(1 + 72) + /* [1] Party A signature and length prefix */
+		(1 + 72) + /* [2] Party B signature and length prefix */
+		(1 + 1 + /* [3] length prefix and numpushes (2) */
+		 33 + /* pubkey A (missing prefix) */
+		 33 + /* pubkey B (missing prefix) */
+		 1 + 1 /* num sigs required and checkmultisig */
+		);
 }
 
 struct amount_sat change_amount(struct amount_sat excess, u32 feerate_perkw,