f more test docs

Author: TheBlueMatt

lightning/src/ln/functional_tests.rs

@@ -9644,8 +9644,13 @@ fn test_dup_htlc_second_fail_panic() {
 		let mut payment_event = SendEvent::from_event(events.pop().unwrap());
 		nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &payment_event.msgs[0]);
 		commitment_signed_dance!(nodes[1], nodes[0], payment_event.commitment_msg, false);
+		// At this point, nodes[1] would notice it has too much value for the payment. It will
+		// assume the second is a privacy attack (no longer particularly relevant
+		// post-payment_secrets) and fail back the new HTLC. Previously, it'd also have failed back
+		// the first HTLC delivered above.
 	}
 
+	// Now we go fail back the first HTLC from the user end.
 	expect_pending_htlcs_forwardable_ignore!(nodes[1]);
 	nodes[1].node.process_pending_htlc_forwards();
 	nodes[1].node.fail_htlc_backwards(&our_payment_hash);