Update and clarify the reasons for HTLCDestination::FailedPayment

Author: TheBlueMatt

lightning/src/events/mod.rs

@@ -232,8 +232,11 @@ pub enum HTLCDestination {
 	///
 	/// Some of the reasons may include:
 	/// * HTLC Timeouts
-	/// * Expected MPP amount has already been reached
-	/// * Claimable amount does not match expected amount
+	/// * Excess HTLCs for a payment that we have already fully received, over-paying for the
+	///   payment,
+	/// * The counterparty node modified the HTLC in transit,
+	/// * A probing attack where an intermediary node is trying to detect if we are the ultimate
+	///   recipient for a payment.
 	FailedPayment {
 		/// The payment hash of the payment we attempted to process.
 		payment_hash: PaymentHash