Add a payments section to ChannelManager docs

Author: jkczyz

lightning/src/ln/channelmanager.rs

@@ -1360,6 +1360,10 @@ where
 /// # }
 /// ```
 ///
+/// # Payments
+///
+/// TODO
+///
 /// # Persistence
 ///
 /// Implements [`Writeable`] to write out all channel state to disk. Implies [`peer_disconnected`] for