docs: add release note for PendingCommitTicker

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

@@ -101,10 +101,14 @@
 * [Make etcd leader election session
   TTL](https://github.com/lightningnetwork/lnd/pull/6342) configurable.
 
-
 * [Fix race condition in the htlc interceptor unit
   test](https://github.com/lightningnetwork/lnd/pull/6353).
 
+* [A new config option, `pending-commit-interval` is
+  added](https://github.com/lightningnetwork/lnd/pull/6186). This value
+  specifies the maximum duration it allows for a remote peer to respond to a
+  locally initiated commitment update.
+
 ## RPC Server
 
 * [Add value to the field