f monitor_key docs on the newly public function

Author: domZippilli

lightning/src/util/persist.rs

@@ -431,6 +431,15 @@ where
 	/// [`io::ErrorKind::NotFound`] variant correctly. For more information, please see the
 	/// documentation for [`MonitorUpdatingPersister`].
 	///
+	/// For `monitor_key`, channel storage keys be the channel's transaction ID and index, or
+	/// [`OutPoint`], with an underscore `_` between them. For example, given:
+	///
+	///   - Transaction ID: `deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef`
+	///   - Index: `1`
+	///
+	/// The correct `monitor_key` would be:
+	/// `deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef_1`
+	/// 
 	/// Loading a large number of monitors will be faster if done in parallel. You can use this
 	/// function to accomplish this. Take care to limit the number of parallel readers.
 	pub fn read_channel_monitor_with_updates<B: Deref, F: Deref + Clone>(