doc: add another signing flow for multisig with descriptor wallets and PSBTs

Author: mjdietzx

doc/descriptors.md

@@ -162,6 +162,11 @@ The basic steps are:
      then the resulting transaction is broadcasted to the network
   8. Checks that balances are correct after the transaction has been included in a block
 
+You may prefer a daisy chained signing flow where each participant signs the PSBT one after another until
+the PSBT has been signed `M` times and is "complete." For the most part, the steps above remain the same, except (6, 7)
+change slightly from signing the original PSBT in parallel to signing it in series. `combinepsbt` is not necessary with
+this signing flow and the last (`m`th) signer can just broadcast the PSBT after signing. Note that a parallel signing flow may be
+preferable in cases where there are more signers. This signing flow is also included in the test / Python example.
 [The test](/test/functional/wallet_multisig_descriptor_psbt.py) is meant to be documentation as much as it is a functional test, so
 it is kept as simple and readable as possible.
 

test/functional/wallet_multisig_descriptor_psbt.py

@@ -139,5 +139,21 @@ def run_test(self):
         assert_approx(self.nodes[0].get_wallet_rpc(f"{self.name}_{0}").getbalance(), deposit_amount - value, vspan=0.001)
         assert_equal(self.nodes[self.N - 1].get_wallet_rpc(f"participant_{self.N - 1}").getbalance(), value)
 
+        self.log.info("Send another transaction from the multisig, this time with a daisy chained signing flow (one after another in series)!")
+        psbt = self.make_sending_transaction(to, value)
+        for m in range(self.M):
+            signing_wallet = self.nodes[m].get_wallet_rpc(f"participant_{m}")
+            psbt = signing_wallet.walletprocesspsbt(psbt["psbt"])
+            assert_equal(psbt["complete"], m == self.M - 1)
+        finalized = coordinator_wallet.finalizepsbt(psbt["psbt"])
+        coordinator_wallet.sendrawtransaction(finalized["hex"])
+
+        self.log.info("Check that balances are correct after the transaction has been included in a block.")
+        self.nodes[0].generate(1)
+        self.sync_all()
+        assert_approx(self.nodes[0].get_wallet_rpc(f"{self.name}_{0}").getbalance(), deposit_amount - (value * 2), vspan=0.001)
+        assert_equal(self.nodes[self.N - 1].get_wallet_rpc(f"participant_{self.N - 1}").getbalance(), value * 2)
+
+
 if __name__ == "__main__":
     WalletMultisigDescriptorPSBTTest().main()