Reference payjoin::io relay guidance in cli doc

Author: bc1cindy

payjoin-cli/src/app/v2/ohttp.rs

@@ -1,5 +1,7 @@
 //! OHTTP relay selection and key bootstrapping for the payjoin-cli.
 //!
+//! See the `payjoin::io` module for relay-selection and health-check guidance.
+//!
 //! [`RelayManager`] tracks relays that have failed, excluding them from
 //! future selections for the lifetime of the [`RelayManager`].
 //!