f - document payment_id parameter

Author: jkczyz

lightning/src/offers/refund.rs

@@ -148,6 +148,9 @@ impl<'a, T: secp256k1::Signing> RefundBuilder<'a, T> {
 	/// Also, sets the metadata when [`RefundBuilder::build`] is called such that it can be used to
 	/// verify that an [`InvoiceRequest`] was produced for the refund given an [`ExpandedKey`].
 	///
+	/// The `payment_id` is encrypted in the metadata and should be unique. This ensures that only
+	/// one invoice will be paid for the refund and that payments can be uniquely identified.
+	///
 	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
 	/// [`ExpandedKey`]: crate::ln::inbound_payment::ExpandedKey
 	pub fn deriving_payer_id<ES: Deref>(