Add pending changelog for Event::PaymentFailed

Author: jkczyz

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

@@ -0,0 +1,12 @@
+## API Updates
+ * `Event::PaymentFailed` now uses an `Option` for its payment hash, which will
+   be `None` if the invoice hasn't been received yet for a BOLT12 payment
+   (#3192).
+ * `Event::PaymentFailed` is now generated instead `Event::InvoiceRequestFailed`,
+   which has been removed. When deserialized the latter will be converted to the
+   former with `None` for the payment hash (#3192).
+
+## Backwards Compatibility
+ * 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).