f add test comment

Author: TheBlueMatt

lightning/src/ln/payment_tests.rs

@@ -2795,6 +2795,10 @@ fn do_no_missing_sent_on_midpoint_reload(persist_manager_with_payment: bool) {
 	// Note that we don't get a PaymentPathSuccessful here as we leave the HTLC pending to avoid
 	// the double-claim that would otherwise appear at the end of this test.
 
+	// Ensure that, even after some time, if we restart we still include *something* in the current
+	// `ChannelManager` which prevents a `PaymentFailed` when we see the HTLC again on-chain later.
+	// A naive implementation of the fix here would wipe the pending payments set, causing a
+	// failure event when we restart.
 	for _ in 0..(IDEMPOTENCY_TIMEOUT_TICKS * 2) { nodes[0].node.timer_tick_occurred(); }
 
 	let chan_0_monitor_serialized = get_monitor!(nodes[0], chan_id).encode();