Merge bitcoin/bitcoin#22067: Test and document a basic M-of-N multisig using descriptor wallets and PSBTs

9de0d94508 doc: add disclaimer highlighting shortcomings of the basic multisig example (Michael Dietz)
f9479e4626 test, doc: basic M-of-N multisig minor cleanup and clarifications (Michael Dietz)
e05cd0546a doc: add another signing flow for multisig with descriptor wallets and PSBTs (Michael Dietz)
17dd657300 doc: M-of-N multisig using descriptor wallets and PSBTs, as well as a signing flow (Michael Dietz)
1f20501efc test: add functional test for multisig flow with descriptor wallets and PSBTs (Michael Dietz)

Pull request description:

  Aims to resolve issue https://github.com/bitcoin/bitcoin/issues/21278. I try to follow the steps laanwj outlined there exactly, with the exception of using `combinepsbt` instead of `joinpsbts`. I wrote a functional test to make sure it works as expected before doing the docs, and figured it would also be a good source of documentation. So I kept the test as simple as possible and didn't go crazy with edge-cases and various checks. I do have a lot more test-cases I've written that I will follow up with (either in a separate PR or another commit - lmk if you have a preference), but I want to do it in a way that doesn't bloat this test so it remains useful as a quickstart (unless that's a bad idea)?

ACKs for top commit:
  S3RK:
    Code review ACK 9de0d94. Rspigler's argument convinced me that we should leave the workflow with two wallets. I assume using multisig with external signers is a popular use-case and it's important to keep compatibility.
  laanwj:
    Code and documentation review ACK 9de0d94508

Tree-SHA512: 6c76e787c21f09d8be5eaa11f3ca3eaa4868497824050562bdfb2095c73b90f5e8987a8775119891d6bfde586e3f31ad1b13e4b67b0802e1d23ef050227a1211

test/functional/test_runner.py

@@ -207,6 +207,7 @@ BASE_SCRIPTS = [
     'feature_assumevalid.py',
     'example_test.py',
     'wallet_txn_doublespend.py --legacy-wallet',
+    'wallet_multisig_descriptor_psbt.py',
     'wallet_txn_doublespend.py --descriptors',
     'feature_backwards_compatibility.py --legacy-wallet',
     'feature_backwards_compatibility.py --descriptors',