docs: update release note for payment lifecycle

docs/release-notes/release-notes-0.18.0.md

@@ -37,6 +37,9 @@
   might panic due to empty witness data found in a transaction. More details
   can be found [here](https://github.com/bitcoin/bitcoin/issues/28730).
 
+* [Fixed a case](https://github.com/lightningnetwork/lnd/pull/7503) where it's
+  possible a failed payment might be stuck in pending.
+
 # New Features
 ## Functional Enhancements
 
@@ -90,6 +93,11 @@
   `lnrpc.GetInfoResponse` message along with the `chain` field in the
   `lnrpc.Chain` message have also been deprecated for the same reason.
 
+* The payment lifecycle code has been refactored to improve its maintainablity.
+  In particular, the complexity involved in the lifecycle loop has been
+  decoupled into logical steps, with each step having its own responsibility,
+  making it easier to reason about the payment flow.
+
 ## Breaking Changes
 ## Performance Improvements