Document how we setup an optional onchain key manager

Depending upon the presence of an `eclair-signer.conf` file in Eclair's data directory, and the names of the wallet set in `eclair-signer.conf` and `eclair.conf`, we can have:
- no onchain key manager (which is the default)
- an onchain key manager that is used to generate descriptors through our API but that is not active. This is how you create a new watch-only Bitcoin wallet to be used by Eclair
- an onchain key manager that is used to sign bitcoin wallet transactions

eclair-core/src/main/scala/fr/acinq/eclair/Setup.scala

@@ -121,6 +121,7 @@ class Setup(val datadir: File,
   // early checks
   PortChecker.checkAvailable(serverBindingAddress)
 
+  // load on onchain key manager if an `eclair-signer.conf` is found in Eclair's data directory
   val onChainKeyManager_opt = LocalOnChainKeyManager.load(datadir, NodeParams.hashFromChain(chain))
 
   val (bitcoin, bitcoinChainHash) = {
@@ -247,6 +248,13 @@ class Setup(val datadir: File,
 
       finalPubkey = new AtomicReference[PublicKey](null)
       pubkeyRefreshDelay = FiniteDuration(config.getDuration("bitcoind.final-pubkey-refresh-delay").getSeconds, TimeUnit.SECONDS)
+      // there are 3 possibilities regarding onchain key management:
+      // 1) there is no `eclair-signer.conf` file in Eclair's data directory, Eclair will not manage Bitcoin core keys, and Eclair's API will not return bitcoin core descriptors. This is the default mode.
+      // 2) there is an `eclair-signer.conf` file in Eclair's data directory, but the name of the wallet set in `eclair-signer.conf` does not match the `eclair.bitcoind.wallet` setting in `eclair.conf`.
+      // Eclair will use the wallet set in `eclair.conf` and will not manage Bitcoin core keys (here we don't set an optional onchain key manager in our bitcoin client) BUT its API will return bitcoin core descriptors.
+      // This is how you would create a new bitcoin wallet whose private keys are managed by Eclair.
+      // 3) there is an `eclair-signer.conf` file in Eclair's data directory, and the name of the wallet set in `eclair-signer.conf` matches the `eclair.bitcoind.wallet` setting in `eclair.conf`.
+      // Eclair will assume that this is a watch-only bitcoin wallet that has been created from descriptors generated by Eclair, and will manage its private keys, and here we pass the onchain key manager to our bitcoin client.
       bitcoinClient = new BitcoinCoreClient(bitcoin, if (bitcoin.wallet == onChainKeyManager_opt.map(_.walletName)) onChainKeyManager_opt else None) with OnchainPubkeyCache {
         val refresher: typed.ActorRef[OnchainPubkeyRefresher.Command] = system.spawn(Behaviors.supervise(OnchainPubkeyRefresher(this, finalPubkey, pubkeyRefreshDelay)).onFailure(typed.SupervisorStrategy.restart), name = "onchain-address-manager")