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

@@ -8563,10 +8563,14 @@ macro_rules! create_offer_builder { ($self: ident, $builder: ty) => {
 	///
 	/// Uses [`MessageRouter`] to construct a [`BlindedPath`] for the offer based on the given
 	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. See those docs for
-	/// privacy implications. However, if one is not found, uses a one-hop [`BlindedPath`] with
-	/// [`ChannelManager::get_our_node_id`] as the introduction node instead. In the latter case,
-	/// the node must be announced, otherwise, there is no way to find a path to the introduction
-	/// node in order to send the [`InvoiceRequest`].
+	/// privacy implications.
+	///
+	/// The [`Router`] used to parameterized [`ChannelManager`] may also affect privacy since it
+	/// implements [`MessageRouter`]. If parameterized with [`DefaultRouter`], it will delegate to
+	/// [`DefaultMessageRouter`]. In that case, it may return a one-hop [`BlindedPath`] with
+	/// [`ChannelManager::get_our_node_id`] as the introduction node if no other path can be found
+	/// *and* the node is announced. Otherwise, there is no way to find a path to the introduction
+	/// in order to send the [`InvoiceRequest`].
 	///
 	/// Also, uses a derived signing pubkey in the offer for recipient privacy.
 	///
@@ -8631,10 +8635,14 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 	///
 	/// Uses [`MessageRouter`] to construct a [`BlindedPath`] for the refund based on the given
 	/// `absolute_expiry` according to [`MAX_SHORT_LIVED_RELATIVE_EXPIRY`]. See those docs for
-	/// privacy implications. 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
-	/// node in order to send the [`Bolt12Invoice`].
+	/// privacy implications.
+	///
+	/// The [`Router`] used to parameterized [`ChannelManager`] may also affect privacy since it
+	/// implements [`MessageRouter`]. If parameterized with [`DefaultRouter`], it will delegate to
+	/// [`DefaultMessageRouter`]. In that case, it may return a one-hop [`BlindedPath`] with
+	/// [`ChannelManager::get_our_node_id`] as the introduction node if no other path can be found
+	/// *and* the node is 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.
 	///