test, doc: basic M-of-N multisig minor cleanup and clarifications

wallet_multisig_descriptor_psbt.py is refactored in this commit. While
behavior doesn't change we do cleanup the way wallets are accessed
throughout the test as this is done a lot for the various signers
and their multisigs. We also get rid of some shallow methods and
instead inline them for improved readability.

descriptors.md is improved to be more explicit about which wallet
(ie the signer or multisig) is required for each step.

doc/descriptors.md

@@ -145,22 +145,24 @@ For a good example of a basic M-of-N multisig between multiple participants usin
 wallets and PSBTs, as well as a signing flow, see [this functional test](/test/functional/wallet_multisig_descriptor_psbt.py).
 The basic steps are:
 
-  1. Every participant generates an xpub. The most straightforward way is to create a new descriptor wallet.
-     Avoid reusing this wallet for any other purpose. Hint: extract the wallet's xpubs using `listdescriptors`
-     and pick the one from the `pkh` descriptor since it's least likely to be accidentally reused (legacy addresses)
+  1. Every participant generates an xpub. The most straightforward way is to create a new descriptor wallet which we will refer to as
+     the participant's signer wallet. Avoid reusing this wallet for any purpose other than signing transactions from the
+     corresponding multisig we are about to create. Hint: extract the wallet's xpubs using `listdescriptors` and pick the one from the
+     `pkh` descriptor since it's least likely to be accidentally reused (legacy addresses)
   2. Create a watch-only descriptor wallet (blank, private keys disabled). Now the multisig is created by importing the two descriptors:
      `wsh(sortedmulti(<M>,XPUB1/0/*,XPUB2/0/*,…,XPUBN/0/*))` and `wsh(sortedmulti(<M>,XPUB1/1/*,XPUB2/1/*,…,XPUBN/1/*))`
      (one descriptor w/ `0` for receiving addresses and another w/ `1` for change). Every participant does this
   3. A receiving address is generated for the multisig. As a check to ensure step 2 was done correctly, every participant
      should verify they get the same addresses
   4. Funds are sent to the resulting address
-  5. A sending transaction is created using `walletcreatefundedpsbt` (anyone can initiate this). It is simple to do this in
-     the GUI by going to the `Send` tab in the multisig wallet and creating an unsigned transaction (PSBT)
-  6. At least `M` users check the PSBT with `decodepsbt` and (if OK) signs it with `walletprocesspsbt`. It is simple to do
-     this in the GUI by Loading the PSBT from file and signing it
-  7. The signed PSBTs are collected with `combinepsbt`, finalized w/ `finalizepsbt`, and
-     then the resulting transaction is broadcasted to the network
-  8. Checks that balances are correct after the transaction has been included in a block
+  5. A sending transaction from the multisig is created using `walletcreatefundedpsbt` (anyone can initiate this). It is simple to do
+     this in the GUI by going to the `Send` tab in the multisig wallet and creating an unsigned transaction (PSBT)
+  6. At least `M` participants check the PSBT with their multisig using `decodepsbt` to verify the transaction is OK before signing it.
+  7. (If OK) the participant signs the PSBT with their signer wallet using `walletprocesspsbt`. It is simple to do this in the GUI by
+     loading the PSBT from file and signing it
+  8. The signed PSBTs are collected with `combinepsbt`, finalized w/ `finalizepsbt`, and then the resulting transaction is broadcasted
+     to the network. Note that any wallet (eg one of the signers or multisig) is capable of doing this.
+  9. 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)