Add docs when RecentPaymentDetails::Fulfilled payment_hash in None

Author: jurvis

lightning/src/ln/channelmanager.rs

@@ -1122,7 +1122,8 @@ pub enum RecentPaymentDetails {
 	/// been resolved. Upon receiving [`Event::PaymentSent`], we delay for a few minutes before the
 	/// payment is removed from tracking.
 	Fulfilled {
-		/// Hash of the payment that was claimed.
+		/// Hash of the payment that was claimed. Returns `None` for possibly older serializations
+		/// of [`ChannelManager`]
 		payment_hash: Option<PaymentHash>,
 	},
 	/// After a payment is explicitly abandoned by calling [`ChannelManager::abandon_payment`], it