doc: document output descriptors in hsmtool

Touch a bit about it as a backup/recovery mechanism in the FAQ

Signed-off-by: Antoine Poinsot <[email protected]>

Author: darosior

doc/FAQ.md

@@ -107,8 +107,13 @@ it can recover all its funds.
 C-lightning has an internal bitcoin wallet, which you can use to make "on-chain"
 transactions, (see [withdraw](https://lightning.readthedocs.io/lightning-withdraw.7.html).
 These on-chain funds are backed up via the HD wallet seed, stored in byte-form in `hsm_secret`.
+The [`hsmtool`](https://lightning.readthedocs.io/lightning-hsmtool.8.html) can be used to
+backup the onchain wallet as [`output descriptors`](https://github.com/bitcoin/bitcoin/blob/master/doc/descriptors.md)
+with the `dumponchaindescriptors` command (with the `private` parameter).
+These descriptors may then be recovered on any wallet supporting output descriptors, for
+example on `bitcoind` by using the `importmulti` or (introduced in `0.21`)
+`importdescriptors` RPC commands.
 
-and which you can backup thanks to a seed stored in the `hsm_secret`.
 `lightningd` also stores information for funds locked in Lightning Network channels, which are stored
 in a database. This database is required for on-going channel updates as well as channel closure.
 There is no single-seed backup for funds locked in channels.

doc/lightning-hsmtool.8

@@ -51,6 +51,18 @@ and is usually no greater than the number of channels that the node has
 ever had\.
 Specify \fIpassword\fR if the \fBhsm_secret\fR is encrypted\.
 
+
+ \fBdumponchaindescriptors\fR \fIhsm_secret\fR [\fIpassword\fR] [\fIprivate\fR] [\fItestnet\fR]
+Dump output descriptors for our onchain wallet\.
+The descriptors can be either expressed using \fIprivate\fR keys, for use as a back up and
+recovery mechanism (for example on \fBbitcoind\fR using the \fBimportmulti\fR or
+\fBimportdescriptors\fR RPC calls), or public for external services to be able to generate
+addresses for our onchain wallet\.
+We need the path to the hsm_secret containing the wallet seed, and an optional (skip using
+\fB""\fR) password if it was encrypted\.
+To generate descriptors using testnet master keys, you may specify \fItestnet\fR as the last
+parameter\. By default, mainnet-encoded keys are used\.
+
 .SH BUGS
 
 You should report bugs on our github issues page, and maybe submit a fix
@@ -76,4 +88,4 @@ Note: the modules in the ccan/ directory have their own licenses, but
 the rest of the code is covered by the BSD-style MIT license\.
 Main web site: \fIhttps://github.com/ElementsProject/lightning\fR
 
-\" SHA256STAMP:11b3e6c050eb31e7faac10e8b75c3f7b7d57269f7f8c47c67f6d115b7a479c09
+\" SHA256STAMP:f2bc7ee339a01e6317593485778ec2a9c0cf11e1e6e26b7e44deb9cfdcedd987

doc/lightning-hsmtool.8.md

@@ -48,6 +48,17 @@ and is usually no greater than the number of channels that the node has
 ever had.
 Specify *password* if the `hsm_secret` is encrypted.
 
+ **dumponchaindescriptors** *hsm_secret* \[*password*\] \[*private*\] \[*testnet*\]
+Dump output descriptors for our onchain wallet.
+The descriptors can be either expressed using *private* keys, for use as a back up and
+recovery mechanism (for example on `bitcoind` using the `importmulti` or
+`importdescriptors` RPC calls), or public for external services to be able to generate
+addresses for our onchain wallet.
+We need the path to the hsm_secret containing the wallet seed, and an optional (skip using
+`""`) password if it was encrypted.
+To generate descriptors using testnet master keys, you may specify *testnet* as the last
+parameter. By default, mainnet-encoded keys are used.
+
 BUGS
 ----