Monitor update failure can no longer lead to a `ChannelUnavailable`
error, but more common cases such as the peer being disconnected
always could, so those should be documented clearer.
/// In general, a path may raise:
/// * [`APIError::InvalidRoute`] when an invalid route or forwarding parameter (cltv_delta, fee,
/// node public key) is specified.
- /// * [`APIError::ChannelUnavailable`] if the next-hop channel is not available for updates
- /// (including due to previous monitor update failure or new permanent monitor update
- /// failure).
+ /// * [`APIError::ChannelUnavailable`] if the next-hop channel is not available as it has been
+ /// closed, doesn't exist, or the peer is currently disconnected.
/// * [`APIError::MonitorUpdateInProgress`] if a new monitor update failure prevented sending the
/// relevant updates.
///