doc: document output descriptors in hsmtool

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

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

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

@@ -55,6 +55,17 @@ Specify \fIpassword\fR if the \fBhsm_secret\fR is encrypted\.
 \fBgeneratehsm\fR \fIhsm_secret_path\fR
 Generates a new hsm_secret using BIP39\.
 
+
+ \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
@@ -80,4 +91,4 @@ 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:918981692d3840344e15c539b007b473d5ea0ad481145eccff092bf61ec6ddb0
+\" SHA256STAMP:3d847c486363271e0635336caca4fd14f5007a3ff463c223fb5bdb52dbf7b98e

doc/lightning-hsmtool.8.md

@@ -51,6 +51,16 @@ Specify *password* if the `hsm_secret` is encrypted.
 **generatehsm** *hsm\_secret\_path*
 Generates a new hsm_secret using BIP39.
 
+ **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
 ----