docs: add release notes for wallet_synced RPC field

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

@@ -116,6 +116,13 @@
 * routerrpc HTLC event subscribers now receive specific failure details for
   invoice-level validation failures, avoiding ambiguous `UNKNOWN` results. [#10520](https://github.com/lightningnetwork/lnd/pull/10520)
 
+* [A new `wallet_synced` field has been
+  added](https://github.com/lightningnetwork/lnd/pull/10507) to the `GetInfo`
+  RPC response. This field indicates whether the wallet is fully synced to the
+  best chain, providing the wallet's internal sync state independently from the
+  composite `synced_to_chain` field which also considers router and blockbeat
+  dispatcher states.
+
 ## lncli Updates
 
 ## Breaking Changes