f - add comment

Author: jkczyz

lightning/src/offers/invoice_request.rs

@@ -1004,13 +1004,16 @@ pub(super) const INVOICE_REQUEST_TYPES: core::ops::Range<u64> = 80..160;
 /// [`Refund::payer_id`]: crate::offers::refund::Refund::payer_id
 pub(super) const INVOICE_REQUEST_PAYER_ID_TYPE: u64 = 88;
 
+// This TLV stream is used for both InvoiceRequest and Refund, but not all TLV records are valid for
+// InvoiceRequest as noted below.
 tlv_stream!(InvoiceRequestTlvStream, InvoiceRequestTlvStreamRef, INVOICE_REQUEST_TYPES, {
 	(80, chain: ChainHash),
 	(82, amount: (u64, HighZeroBytesDroppedBigSize)),
 	(84, features: (InvoiceRequestFeatures, WithoutLength)),
 	(86, quantity: (u64, HighZeroBytesDroppedBigSize)),
 	(INVOICE_REQUEST_PAYER_ID_TYPE, payer_id: PublicKey),
 	(89, payer_note: (String, WithoutLength)),
+	// Only used for Refund since the onion message of an InvoiceRequest has a reply path.
 	(90, paths: (Vec<BlindedPath>, WithoutLength)),
 });