Document when PaymentPathSuccessful::payment_hash is filled in.

The `payment_hash` field in `PaymentPathSuccessful` is always
`Some` as long as the pening payment tracker has a `payment_hash`,
which is true for all `Pending` payments as well as all `Fulfilled`
payments starting with the commit which added
`PaymentPathSuccessful` -
3b5c370b404e2f5a8f3c35093b97406f149a9340c177c05252574083d68df0da.

Author: TheBlueMatt

lightning/src/events/mod.rs

@@ -498,6 +498,8 @@ pub enum Event {
 		payment_id: PaymentId,
 		/// The hash that was given to [`ChannelManager::send_payment`].
 		///
+		/// This will be `Some` for all payments which completed on LDK 0.0.104 or later.
+		///
 		/// [`ChannelManager::send_payment`]: crate::ln::channelmanager::ChannelManager::send_payment
 		payment_hash: Option<PaymentHash>,
 		/// The payment path that was successful.

lightning/src/ln/outbound_payment.rs

@@ -60,6 +60,7 @@ pub(crate) enum PendingOutboundPayment {
 	/// and add a pending payment that was already fulfilled.
 	Fulfilled {
 		session_privs: HashSet<[u8; 32]>,
+		/// Filled in for any payment which moved to `Fulfilled` on LDK 0.0.104 or later.
 		payment_hash: Option<PaymentHash>,
 		timer_ticks_without_htlcs: u8,
 	},
@@ -1168,9 +1169,11 @@ impl OutboundPayments {
 				if let hash_map::Entry::Occupied(mut payment) = outbounds.entry(payment_id) {
 					assert!(payment.get().is_fulfilled());
 					if payment.get_mut().remove(&session_priv_bytes, None) {
+						let payment_hash = payment.get().payment_hash();
+						debug_assert!(payment_hash.is_some());
 						pending_events.push_back((events::Event::PaymentPathSuccessful {
 							payment_id,
-							payment_hash: payment.get().payment_hash(),
+							payment_hash,
 							path,
 						}, None));
 					}