Merge bitcoin/bitcoin#36136: rpc: remove stale "canonical form" claim from getdescriptorinfo help

Merge bitcoin/bitcoin#36136: rpc: remove stale "canonical form" claim from getdescriptorinfo help

1dad06eff3d564a56b72d35b7984ce868957daa7 remove stale canonical form claim from getdescriptorinfo help (Craig Raw)

Pull request description:

`getdescriptorinfo` describes its `descriptor` result as:

> The descriptor in canonical form, without private keys.

The returned string is a re-serialisation of the parsed descriptor with private keys removed. It is not a canonical form: descriptors that describe the same wallet routinely come back as different strings with different checksums, and this is deliberate.

Three things the RPC does not canonicalise:

- **The hardened derivation marker.** #26076 added `m_apostrophe` so that the marker the caller used is preserved rather than rewritten (`src/script/descriptor.cpp:262`, `:512`); it first shipped in v26.0. Before that, `FormatHDKeypath()` emitted `'` unconditionally (v25.0, `src/util/bip32.cpp:54`), so `h` supplied by the caller was rewritten — the behaviour #15740 objected to under the name "canonicalize". Where a single key expression mixes both markers, the style of its last hardened element is applied to the whole expression, which is still input-dependent.
- **Key order in `multi()` / `sortedmulti()`.** The order is preserved as given. For `sortedmulti()` the written order carries no meaning at all, since BIP 383 sorts the derived keys when the output script is built, so the same wallet has n! equally valid descriptors.
- **The checksum.** The `checksum` field is computed over the *input* string (`src/rpc/output_script.cpp:215`), so it can differ from the checksum embedded in the returned `descriptor` — e.g. when a private key was supplied.

```
$ bitcoin-cli getdescriptorinfo "sortedmulti(1,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235)" | jq -r .descriptor
sortedmulti(1,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd,04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235)#fne5696l

$ bitcoin-cli getdescriptorinfo "sortedmulti(1,04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd)" | jq -r .descriptor
sortedmulti(1,04a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd5b8dec5235a0fa8722476c7709c02559e3aa73aa03918ba2d492eea75abea235,03a34b99f22c790c4e36b2b3c2c35a36db06226e41c692fc82b8b56ac1c540c5bd)#w5gau8hw
```

Both produce the same output script; neither is more canonical than the other. The same applies to the marker:

```
$ bitcoin-cli getdescriptorinfo "wpkh([f6bb4c63/0h/0h/30h]028429a37c3f09c8c5cc1fab58df32d1a7da7616c748a40eeb1aae1d64acb9c5cc)" | jq -r .descriptor
wpkh([f6bb4c63/0h/0h/30h]028429a37c3f09c8c5cc1fab58df32d1a7da7616c748a40eeb1aae1d64acb9c5cc)#vk9vfu0h

$ bitcoin-cli getdescriptorinfo "wpkh([f6bb4c63/0'/0'/30']028429a37c3f09c8c5cc1fab58df32d1a7da7616c748a40eeb1aae1d64acb9c5cc)" | jq -r .descriptor
wpkh([f6bb4c63/0'/0'/30']028429a37c3f09c8c5cc1fab58df32d1a7da7616c748a40eeb1aae1d64acb9c5cc)#5wdxpxcx
```

The wording dates from v0.18.0 (`src/rpc/misc.cpp:153`), where it did describe the behaviour, and has been carried forward unchanged since. Whether the RPC should canonicalise was settled in #15740 in favour of round-tripping what the caller supplied; this only brings the description into line with that outcome.

I have deliberately not replaced the phrase with "normal form" or "normalized". Those terms already denote a different transformation in this codebase — BIP 380's "Normalization of Key Expressions with Hardened Derivation", implemented as `Descriptor::ToNormalizedString()` ("Normalized descriptors have the xpub at the last hardened step", `src/script/descriptor.h:140`) — which `getdescriptorinfo` does not perform. Dropping the qualifier avoids the collision.

Documentation only; no behaviour change. The phrase occurs nowhere else in the repo.

```diff
-{RPCResult::Type::STR, "descriptor", "The descriptor in canonical form, without private keys. For a multipath descriptor, only the first will be returned."},
+{RPCResult::Type::STR, "descriptor", "The descriptor, without private keys. For a multipath descriptor, only the first will be returned."},
```

If reviewers would rather the help positively state what is preserved (hardened marker and key order as supplied, checksum recomputed), I am happy to expand it; I have kept the change minimal.

ACKs for top commit:
l0rinc:
ACK 1dad06eff3d564a56b72d35b7984ce868957daa7
Eunovo:
ACK https://github.com/bitcoin/bitcoin/pull/36136/commits/1dad06eff3d564a56b72d35b7984ce868957daa7:
rkrux:
lgtm ACK 1dad06eff3d564a56b72d35b7984ce868957daa7

Tree-SHA512: 7b03100fdbc71c867d26094eb3975dfe967875a977ef3c7264b8bb3b920db319d6f11dbde1d39bb70fff2b2288eca42aadffc215ab7503dcc7b3bf62fc2692e5