f expand docs

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

Author: valentinewallace

lightning-invoice/src/payment.rs

@@ -209,6 +209,9 @@ where
 ///
 /// You should ensure that the [`PaymentPreimage`] is unique and the corresponding
 /// [`PaymentHash`] has never been paid before.
+///
+/// See [`pay_pubkey`] for a variant which uses the [`PaymentHash`] for the
+/// idempotency token.
 pub fn pay_pubkey_with_id<M: Deref, T: Deref, ES: Deref, NS: Deref, SP: Deref, F: Deref, R: Deref, L: Deref>(
 	pubkey: PublicKey, payment_preimage: PaymentPreimage, payment_id: PaymentId,
 	amount_msats: u64, final_cltv_expiry_delta: u32, retry_strategy: Retry,