add some doc comments

backend/common/src/internal_messages.rs

@@ -1,3 +1,5 @@
+//! Internal messages passed between the shard and telemetry core.
+
 use std::net::IpAddr;
 
 use crate::id_type;

backend/common/src/log_level.rs

@@ -1,5 +1,6 @@
-/// A log level that can be used as an expected argument,
+/// A log level that can be decoded from a string (and so can be used
-/// decoded from a string, and converted into a [`log::LevelFilter`]
+/// as a CLI argument by things like StructOpt). It can be converted into
+/// a [`log::LevelFilter`] for passing into logging systems.
 #[derive(Debug, PartialEq)]
 pub enum LogLevel {
     Error,

backend/common/src/node_message.rs

@@ -1,3 +1,8 @@
+//! This is the internal represenation of telemetry messages sent from nodes.
+//! There is a separate JSON representation of these types, because internally we want to be
+//! able to serialize these messages to bincode, and various serde attribtues aren't compatible
+//! with this, hence this separate internal representation.
+
 use crate::node_types::{Block, BlockHash, BlockNumber, NodeDetails};
 use serde::{Deserialize, Serialize};
 

backend/common/src/node_types.rs

@@ -1,3 +1,6 @@
+//! These types are partly used in [`crate::node_message`], but also stored and used
+//! more generally through the application.
+
 use serde::ser::{SerializeTuple, Serializer};
 use serde::{Deserialize, Serialize};