docs: describe remote signing upgrade process

This commit adds a short guide that explains how a remote signing node
setup should be migrated from lnd v0.14.x-beta to lnd v0.15.x-beta and
adds a note to all 0.15.x release notes.

docs/release-notes/release-notes-0.15.0.md

@@ -34,6 +34,12 @@ were created after introducing the Taproot key
 derivation](https://github.com/lightningnetwork/lnd/pull/6524) to simplify
 detecting Taproot compatibility of a seed.
 
+**NOTE** for users running a remote signing setup: A manual account import is
+necessary when upgrading from `lnd v0.14.x-beta` to `lnd v0.15.x-beta`, see [the
+remote signing documentation for more
+details](../remote-signing.md#migrating-a-remote-signing-setup-from-014x-to-015x).
+Please upgrade to `lnd v0.15.2-beta` or later directly!
+
 ## MuSig2
 
 The [`signrpc.Signer` RPC service now supports EXPERIMENTAL MuSig2

docs/release-notes/release-notes-0.15.1.md

@@ -44,6 +44,12 @@ supports the feature.](https://github.com/lightningnetwork/lnd/pull/6633)
 The [wallet also creates P2TR change addresses by
 default](https://github.com/lightningnetwork/lnd/pull/6810) in most cases.
 
+**NOTE** for users running a remote signing setup: A manual account import is
+necessary when upgrading from `lnd v0.14.x-beta` to `lnd v0.15.x-beta`, see [the
+remote signing documentation for more
+details](../remote-signing.md#migrating-a-remote-signing-setup-from-014x-to-015x).
+Please upgrade to `lnd v0.15.2-beta` or later directly!
+
 ## `lncli`
 
 * [Add `payment_addr` flag to

docs/release-notes/release-notes-0.15.2.md

@@ -14,15 +14,19 @@
 * [A bug has been fixed that caused fee estimation to be incorrect for taproot
   inputs when using the `SendOutputs` call.](https://github.com/lightningnetwork/lnd/pull/6941)
 
-# Contributors (Alphabetical Order)
+## Taproot
 
 
+**NOTE** for users running a remote signing setup: A manual account import is
+necessary when upgrading from `lnd v0.14.x-beta` to `lnd v0.15.x-beta`, see [the
+remote signing documentation for more
+details](../remote-signing.md#migrating-a-remote-signing-setup-from-014x-to-015x).
+
 ## Performance improvements
 
 * [Refactor hop hint selection
   algorithm](https://github.com/lightningnetwork/lnd/pull/6914)
 
-
 # Contributors (Alphabetical Order)
 
 * Eugene Siegel

docs/remote-signing.md

@@ -153,6 +153,28 @@ To migrate an existing node, follow these steps:
    a watch-only one (by purging all private key material from it) by adding the
   `remotesigner.migrate-wallet-to-watch-only=true` configuration entry.
 
+## Migrating a remote signing setup from 0.14.x to 0.15.x
+
+If you were running a remote signing setup with `lnd v0.14.x-beta` and want to
+upgrade to `lnd v0.15.x-beta`, you need to manually import the newly added
+Taproot account to the watch-only node, otherwise you will encounter errors such
+as `account 0 not found` when doing on-chain operations that require creating
+(change) P2TR addresses.
+
+**NOTE**: For this to work, you need to upgrade to at least `lnd v0.15.2-beta`
+or later!
+
+The upgrade process should look like this:
+1. Upgrade the "signer" node to `lnd v0.15.x-beta` and unlock it.
+2. Run `lncli wallet accounts list | grep -A5 TAPROOT` on the **"signer"** node
+   and copy the `xpub...` value from `extended_public_key`.
+3. Upgrade the "watch-only" node to `lnd v0.15.x-beta` and unlock it.
+4. Run `lncli wallet accounts import --address_type p2tr <xpub...> default` on
+   the **"watch-only"** node (notice the `default` account name at the end,
+   that's important).
+5. Run `lncli newaddress p2tr` on the "watch-only" node to test that everything
+   works as expected.
+
 ## Example initialization script
 
 This section shows an example script that initializes the watch-only wallet of