docs: add release notes entry

Author: starius

docs/release-notes/release-notes-0.20.0.md

@@ -59,6 +59,11 @@
 - [Fixed a bug](https://github.com/lightningnetwork/lnd/pull/10330) to ensure that goroutine resources are properly freed in the case
   of a disconnection or other failure event.
 
+- Chain notifier RPCs now [return the gRPC `Unavailable`
+  status](https://github.com/lightningnetwork/lnd/pull/10352) while the
+  sub-server is still starting. This allows clients to reliably detect the
+  transient condition and retry without brittle string matching.
+
 # New Features
 
 * Use persisted [nodeannouncement](https://github.com/lightningnetwork/lnd/pull/8825)