Document ser for BOLT 12 pending outbound payments.

Useful since these variants will be dropped on downgrade, so want some text to
explain why that's okay.

lightning/src/ln/outbound_payment.rs

@@ -2063,17 +2063,23 @@ impl_writeable_tlv_based_enum_upgradable!(PendingOutboundPayment,
 		(1, reason, option),
 		(2, payment_hash, required),
 	},
+	// Added in 0.0.123. Prior versions will drop these outbounds on downgrade, which is safe because
+	// no HTLCs are in-flight.
 	(5, AwaitingInvoice) => {
 		(0, expiration, required),
 		(1, invoice_request, option),
 		(2, retry_strategy, required),
 		(4, max_total_routing_fee_msat, option),
 	},
+	// Added in 0.0.123. Prior versions will drop these outbounds on downgrade, which is safe because
+	// no HTLCs are in-flight.
 	(7, InvoiceReceived) => {
 		(0, payment_hash, required),
 		(2, retry_strategy, required),
 		(4, max_total_routing_fee_msat, option),
 	},
+	// Added in 0.0.124. Prior versions will drop these outbounds on downgrade, which is safe because
+	// no HTLCs are in-flight.
 	(9, StaticInvoiceReceived) => {
 		(0, payment_hash, required),
 		(2, keysend_preimage, required),