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]>

doc/FAQ.md

@@ -108,7 +108,6 @@ 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`.
 
-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 +51,17 @@ 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] [\fInetwork\fR]
+Dump output descriptors for our onchain wallet\.
+The descriptors can be used by external services to be able to generate
+addresses for our onchain wallet\. (for example on \fBbitcoind\fR using the
+\fBimportmulti\fR or \fBimportdescriptors\fR RPC calls)
+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 generated\.
+
 .SH BUGS
 
 You should report bugs on our github issues page, and maybe submit a fix
@@ -76,4 +87,4 @@ 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:f45b2cc12afaa97f257941056563042867ad86d4dfe5dbe624710042c1466bee

doc/lightning-hsmtool.8.md

@@ -48,6 +48,16 @@ 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* \[*password*\] \[*network*\]
+Dump output descriptors for our onchain wallet.
+The descriptors can be used by external services to be able to generate
+addresses for our onchain wallet. (for example on `bitcoind` using the
+`importmulti` or `importdescriptors` RPC calls)
+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 generated.
+
 BUGS
 ----