Update pending changelog with PaymentFailureReason

Author: jkczyz

pending_changelog/3192-invoice-request-failed-event.txt

@@ -10,3 +10,7 @@
  * Any `Event::PaymentFailed` generated without a payment hash will deserialize
    with `PaymentHash([0; 32])` when downgrading. This can be treated like an
    `Event::InvoiceRequestFailed` (#3192).
+ * An `Event::PaymentFailed` generated with one of the following
+   `PaymentFailureReason`s will fail to deserialize after downgrading:
+   `UnknownRequiredFeatures`, `InvoiceRequestExpired`, or
+   `InvoiceRequestRejected` (#3192).