f expand docs

Co-authored-by: dunxen <[email protected]>

Author: valentinewallace

lightning-invoice/src/payment.rs

@@ -176,6 +176,9 @@ fn pay_invoice_using_amount<P: Deref>(
 /// The hash of the [`PaymentPreimage`] is used as the [`PaymentId`], which ensures idempotency
 /// as long as the payment is still pending. Once the payment completes or fails, you must
 /// ensure that a second payment with the same [`PaymentPreimage`] is never sent.
+///
+/// If you wish to use a different payment idempotency token, see
+/// [`pay_pubkey_with_id`].
 pub fn pay_pubkey<M: Deref, T: Deref, ES: Deref, NS: Deref, SP: Deref, F: Deref, R: Deref, L: Deref>(
 	pubkey: PublicKey, payment_preimage: PaymentPreimage, amount_msats: u64,
 	final_cltv_expiry_delta: u32, retry_strategy: Retry,