docs: add release note for actor SingletonRef

Author: gijswijs

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

@@ -423,6 +423,16 @@
   `lnutils.ContextFromQuit` helper bridges the existing `quit` channels to
   `context.Context`, so all gossip awaits now respect shutdown uniformly.
 
+* [Added `SingletonRef` to the `actor`
+  package](https://github.com/lightningnetwork/lnd/pull/10759) for service
+  keys that are expected to have at most one registered actor.
+  `ServiceKey.SpawnSingleton` and `ServiceKey.Singleton` give spawners and
+  consumers a direct, type-safe path to a single-actor service, replacing
+  the `Router + RoundRobinStrategy` pattern previously needed to front a
+  singleton. `SingletonRef` tolerates transient double-registration by
+  logging the invariant violation and using the first registered actor, so
+  the system stays functional through brief registration races.
+
 ## Tooling and Documentation
 
 * [Added missing `lncli:` tags](https://github.com/lightningnetwork/lnd/pull/10658)