docs: update release notes 0.15.0

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

@@ -81,6 +81,9 @@ then watch it on chain. Taproot script spends are also supported through the
 * [Fixed a data race in the websocket proxy
   code](https://github.com/lightningnetwork/lnd/pull/6380).
 
+* [Fixed race condition resulting in MPP payments sometimes getting stuck
+  in-flight](https://github.com/lightningnetwork/lnd/pull/6352).
+
 ## Misc
 
 * [An example systemd service file](https://github.com/lightningnetwork/lnd/pull/6033)