f clarify method comment

lightning/src/ln/outbound_payment.rs

@@ -294,6 +294,11 @@ impl PendingOutboundPayment {
  // `ChannelManager` after transitioning from this state to `Retryable` prior to shutdown.
  // Therefore, we need to move this payment to `Retryable` now to avoid double-paying if
  // the recipient sends a duplicate invoice.
+ //
+ // We leave many fields as `None` if they are only used for retries, since we'll never
+ // retry on startup. See this method's callsite in `ChannelManager::read`, but this
+ // mirrors the behavior if an HTLC in a monitor is entirely missing from
+ // `OutboundPayments`.
  *self = Self::Retryable {
  retry_strategy: None,
  attempts: PaymentAttempts::new(),