docs: update v0.21 release notes

In this commit, we add release notes for the switchrpc build
tag behavior and the DisableRemoteRouter RPC.

Author: calvinrzachman

docs/release-notes/release-notes-0.21.0.md

@@ -126,6 +126,24 @@
   as either `DefiniteFailure` (safe to fail the attempt) or `IndefiniteFailure`
   (client MUST retry). This is a **breaking change** for any clients of the
   `SendOnion` RPC.
+  
+* To support scenarios where an external entity, such as a remote router,
+  manages the payment lifecycle via the Switch RPC server, the node must
+  preserve the history of HTLC attempts across restarts. This [behavior](https://github.com/lightningnetwork/lnd/pull/10178) is now
+  conditional on how the lnd binary is built. When compiled with the `switchrpc`
+  build tag, the local `routing.ChannelRouter`'s automatic cleanup of the
+  dispatcher's (Switch) attempt store on startup is disabled. This shifts the
+  responsibility of state cleanup to the external controller, which is expected
+  to use an RPC interface (e.g., switchrpc) to manage the lifecycle of attempts.
+  Tying this behavior to a build tag, rather than a runtime flag, makes the
+  binary's purpose explicit and prevents potential misconfigurations.
+
+* Add [`DisableRemoteRouter` rpc](https://github.com/lightningnetwork/lnd/pull/10178)
+  to `switchrpc` which marks the database as no longer being used by a remote
+  router. This is useful for migrating from a remote router setup back to the
+  default embedded router. The external controller should first clean its
+  results from the attempt store via `DeleteAttempts` (#10602); this RPC will
+  fail if there are any remaining attempt entries.
 
 * The `ChannelRouter` now supports a [configurable attempt reconciliation
   hook](https://github.com/lightningnetwork/lnd/pull/10621) that runs for each