Add descriptor reference documentation

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+# Support for Output Descriptors in Bitcoin Core
+
+Since Bitcoin Core v0.17, there is support for Output Descriptors in the
+`scantxoutset` RPC call. This is a simple language which can be used to
+describe collections of output scripts.
+
+This document describes the language. For the specifics on usage for scanning
+the UTXO set, see the `scantxoutset` RPC help.
+
+## Features
+
+Output descriptors currently support:
+- Pay-to-pubkey scripts (P2PK), through the `pk` function.
+- Pay-to-pubkey-hash scripts (P2PKH), through the `pkh` function.
+- Pay-to-witness-pubkey-hash scripts (P2WPKH), through the `wpkh` function.
+- Pay-to-script-hash scripts (P2SH), through the `sh` function.
+- Pay-to-witness-script-hash scripts (P2WSH), through the `wsh` function.
+- Multisig scripts, through the `multi` function.
+- Any type of supported address through the `addr` function.
+- Raw hex scripts through the `raw` function.
+- Public keys (compressed and uncompressed) in hex notation, or BIP32 extended pubkeys with derivation paths.
+
+## Examples
+
+- `pk(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798)` represents a P2PK output.
+- `pkh(02c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5)` represents a P2PKH output.
+- `wpkh(02f9308a019258c31049344f85f89d5229b531c845836f99b08601f113bce036f9)` represents a P2WPKH output.
+- `sh(wpkh(03fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556))` represents a P2SH-P2WPKH output.
+- `combo(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798)` represents a P2PK, P2PKH, P2WPKH, and P2SH-P2WPKH output.
+- `sh(wsh(pkh(02e493dbf1c10d80f3581e4904930b1404cc6c13900ee0758474fa94abe8c4cd13)))` represents a (overly complicated) P2SH-P2WSH-P2PKH output.
+- `multi(1,022f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,025cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc)` represents a bare *1-of-2* multisig.
+- `sh(multi(2,022f01e5e15cca351daff3843fb70f3c2f0a1bdd05e5af888a67784ef3e10a2a01,03acd484e2f0c7f65309ad178a9f559abde09796974c57e714c35f110dfc27ccbe))` represents a P2SH *2-of-2* multisig.
+- `wsh(multi(2,03a0434d9e47f3c86235477c7b1ae6ae5d3442d49b1943c2b752a68e2a47e247c7,03774ae7f858a9411e5ef4246b70c65aac5649980be5c17891bbec17895da008cb,03d01115d548e7561b15c38f004d734633687cf4419620095bc5b0f47070afe85a))` represents a P2WSH *2-of-3* multisig.
+- `sh(wsh(multi(1,03f28773c2d975288bc7d1d205c3748651b075fbc6610e58cddeeddf8f19405aa8,03499fdf9e895e719cfd64e67f07d38e3226aa7b63678949e6e49b241a60e823e4,02d7924d4f7d43ea965a465ae3095ff41131e5946f3c85f79e44adbcf8e27e080e)))` represents a P2SH-P2WSH *1-of-3* multisig.
+- `pk(xpub661MyMwAqRbcFtXgS5sYJABqqG9YLmC4Q1Rdap9gSE8NqtwybGhePY2gZ29ESFjqJoCu1Rupje8YtGqsefD265TMg7usUDFdp6W1EGMcet8)` refers to a single P2PK output, using the public key part from the specified xpub.
+- `pkh(xpub68Gmy5EdvgibQVfPdqkBBCHxA5htiqg55crXYuXoQRKfDBFA1WEjWgP6LHhwBZeNK1VTsfTFUHCdrfp1bgwQ9xv5ski8PX9rL2dZXvgGDnw/1'/2)` refers to a single P2PKH output, using child key *1'/2* of the specified xpub.
+- `wsh(multi(1,xpub661MyMwAqRbcFW31YEwpkMuc5THy2PSt5bDMsktWQcFF8syAmRUapSCGu8ED9W6oDMSgv6Zz8idoc4a6mr8BDzTJY47LJhkJ8UB7WEGuduB/1/0/*,xpub69H7F5d8KSRgmmdJg2KhpAK8SR3DjMwAdkxj3ZuxV27CprR9LgpeyGmXUbC6wb7ERfvrnKZjXoUmmDznezpbZb7ap6r1D3tgFxHmwMkQTPH/1/0/*))` refers to a chain of *1-of-2* P2WSH multisig outputs, using public keys taken from two HD chains with corresponding derivation paths.
+
+## Reference
+
+Descriptors consist of several types of expressions. The top level expression is always a `SCRIPT`.
+
+`SCRIPT` expressions:
+- `sh(SCRIPT)` (top level only): P2SH embed the argument.
+- `wsh(SCRIPT)` (not inside another 'wsh'): P2WSH embed the argument.
+- `pk(KEY)` (anywhere): P2PK output for the given public key.
+- `pkh(KEY)` (anywhere): P2PKH output for the given public key (use `addr` if you only know the pubkey hash).
+- `wpkh(KEY)` (not inside `wsh`): P2WPKH output for the given compressed pubkey.
+- `combo(KEY)` (top level only): an alias for the collection of `pk(KEY)` and `pkh(KEY)`. If the key is compressed, it also includes `wpkh(KEY)` and `sh(wpkh(KEY))`.
+- `multi(k,KEY_1,KEY_2,...,KEY_n)` (anywhere): k-of-n multisig script.
+- `addr(ADDR)` (top level only): the script which ADDR expands to.
+- `raw(HEX)` (top level only): the script whose hex encoding is HEX.
+
+`KEY` expressions:
+- Hex encoded public keys (66 characters starting with `02` or `03`, or 130 characters starting with `04`).
+  - Inside `wpkh` and `wsh`, only compressed public keys are permitted.
+- [WIF](https://en.bitcoin.it/wiki/Wallet_import_format) encoded private keys may be specified instead of the corresponding public key, with the same meaning.
+-`xpub` encoded extended public key or `xprv` encoded private key (as defined in [BIP 32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki)).
+  - Followed by zero or more `/NUM` unhardened and `/NUM'` hardened BIP32 derivation steps.
+  - Optionally followed by a single `/*` or `/*'` final step to denote all (direct) unhardened or hardened children.
+  - The usage of hardened derivation steps requires providing the private key.
+  - Instead of a `'`, the suffix `h` can be used to denote hardened derivation.
+
+`ADDR` expressions are any type of supported address:
+- P2PKH addresses (base58, of the form `1...`). Note that P2PKH addresses in descriptors cannot be used for P2PK outputs (use the `pk` function instead).
+- P2SH addresses (base58, of the form `3...`, defined in [BIP 13](https://github.com/bitcoin/bips/blob/master/bip-0013.mediawiki)).
+- Segwit addresses (bech32, of the form `bc1...`, defined in [BIP 173](https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki)).
+
+## Explanation
+
+### Single-key scripts
+
+Many single-key constructions are used in practice, generally including
+P2PK, P2PKH, P2WPKH, and P2SH-P2WPKH. Many more combinations are
+imaginable, though they may not be optimal: P2SH-P2PK, P2SH-P2PKH,
+P2WSH-P2PK, P2WSH-P2PKH, P2SH-P2WSH-P2PK, P2SH-P2WSH-P2PKH.
+
+To describe these, we model these as functions. The functions `pk`
+(P2PK), `pkh` (P2PKH) and `wpkh` (P2WPKH) take as input a public key in
+hexadecimal notation (which will be extended later), and return the
+corresponding *scriptPubKey*. The functions `sh` (P2SH) and `wsh` (P2WSH)
+take as input a script, and return the script describing P2SH and P2WSH
+outputs with the input as embedded script. The names of the functions do
+not contain "p2" for brevity.
+
+### Multisig
+
+Several pieces of software use multi-signature (multisig) scripts based
+on Bitcoin's OP_CHECKMULTISIG opcode. To support these, we introduce the
+`multi(k,key_1,key_2,...,key_n)` function. It represents a *k-of-n*
+multisig policy, where any *k* out of the *n* provided public keys must
+sign.
+
+### BIP32 derived keys and chains
+
+Most modern wallet software and hardware uses keys that are derived using
+BIP32 ("HD keys"). We support these directly by permitting strings
+consisting of an extended public key (commonly referred to as an *xpub*)
+plus derivation path anywhere a public key is expected. The derivation
+path consists of a sequence of 0 or more integers (in the range
+*0..2<sup>31</sup>-1*) each optionally followed by `'` or `h`, and
+separated by `/` characters. The string may optionally end with the
+literal `/*` or `/*'` (or `/*h`) to refer to all unhardened or hardened
+child keys instead.
+
+Whenever a public key is described using a hardened derivation step, the
+script cannot be computed without access to the corresponding private
+key.
+
+### Including private keys
+
+Often it is useful to communicate a description of scripts along with the
+necessary private keys. For this reason, anywhere a public key or xpub is
+supported, a private key in WIF format or xprv may be provided instead.
+This is useful when private keys are necessary for hardened derivation
+steps, or for dumping wallet descriptors including private key material.
+
+### Compatibility with old wallets
+
+In order to easily represent the sets of scripts currently supported by
+existing Bitcoin Core wallets, a convenience function `combo` is
+provided, which takes as input a public key, and constructs the P2PK,
+P2PKH, P2WPKH, and P2SH-P2WPH scripts for that key. In case the key is
+uncompressed, it only constructs P2PK and P2PKH.