Clarify docs regarding one-hop blinded paths

The docs assumed ChannelManager is parameterized by DefaultRouter, which
may not be the case. Clarify the behavior is specific to using
DefaultRouter.

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -8558,10 +8558,11 @@ macro_rules! create_offer_builder { ($self: ident, $builder: ty) => {
 	/// # Privacy
 	///
 	/// Uses [`MessageRouter`] to construct a [`BlindedPath`] for the offer based on the given
-	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. However, if one is not
-	/// found, uses a one-hop [`BlindedPath`] with [`ChannelManager::get_our_node_id`] as the
-	/// introduction node instead. In the latter case, the node must be announced, otherwise, there
-	/// is no way to find a path to the introduction in order to send the [`InvoiceRequest`].
+	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. When parameterized by
+	/// [`DefaultRouter`] (and thus using [`DefaultMessageRouter`), however, if a path is not found,
+	/// uses a one-hop [`BlindedPath`] with [`ChannelManager::get_our_node_id`] as the introduction
+	/// node instead. In the latter case, the node must be announced, otherwise, there is no way to
+	/// find a path to the introduction in order to send the [`InvoiceRequest`].
 	///
 	/// Also, uses a derived signing pubkey in the offer for recipient privacy.
 	///
@@ -8625,10 +8626,11 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 	/// # Privacy
 	///
 	/// Uses [`MessageRouter`] to construct a [`BlindedPath`] for the refund based on the given
-	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. However, if one is not
-	/// found, uses a one-hop [`BlindedPath`] with [`ChannelManager::get_our_node_id`] as the
-	/// introduction node instead. In the latter case, the node must be announced, otherwise, there
-	/// is no way to find a path to the introduction in order to send the [`Bolt12Invoice`].
+	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. When parameterized by
+	/// [`DefaultRouter`] (and thus using [`DefaultMessageRouter`), however, if a path is not found,
+	/// uses a one-hop [`BlindedPath`] with [`ChannelManager::get_our_node_id`] as the introduction
+	/// node instead. In the latter case, the node must be announced, otherwise, there is no way to
+	/// find a path to the introduction in order to send the [`Bolt12Invoice`].
 	///
 	/// Also, uses a derived payer id in the refund for payer privacy.
 	///