f - fix/update pay_for_offer docs

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -7144,18 +7144,22 @@ where
 	/// Uses [`InvoiceRequestBuilder`] such that the [`InvoiceRequest`] it builds is recognized by
 	/// the [`ChannelManager`] when handling a [`Bolt12Invoice`] message for the request. The
 	/// optional parameters are used in the builder, if `Some`:
-	/// - `quantity` for [`InvoiceRequest::quantity`] which may be set if
+	/// - `quantity` for [`InvoiceRequest::quantity`] which must be set if
 	///   [`Offer::expects_quantity`] is `true`.
 	/// - `amount_msats` if overpaying what is required for the given `quantity` is desired, and
 	/// - `payer_note` for [`InvoiceRequest::payer_note`].
 	///
-	/// The provided `payment_id` is used to ensure that only one invoice is paid for the request.
+	/// The provided `payment_id` is used to ensure that only one invoice is paid for the request
+	/// when received. See [Avoiding Duplicate Payments] for other requirements once the payment has
+	/// been sent. Errors if a duplicate `payment_id` is provided given the caveats in the
+	/// aforementioned link.
 	///
 	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
 	/// [`InvoiceRequest::quantity`]: crate::offers::invoice_request::InvoiceRequest::quantity
 	/// [`InvoiceRequest::payer_note`]: crate::offers::invoice_request::InvoiceRequest::payer_note
 	/// [`InvoiceRequestBuilder`]: crate::offers::invoice_request::InvoiceRequestBuilder
 	/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
+	/// [Avoiding Duplicate Payments]: #avoiding-duplicate-payments
 	pub fn pay_for_offer(
 		&self, offer: &Offer, quantity: Option<u64>, amount_msats: Option<u64>,
 		payer_note: Option<String>, payment_id: PaymentId, retry_strategy: Retry,