Add doc comments for validator service.

validator_client/src/duties/traits.rs

@@ -6,7 +6,11 @@ pub enum BeaconNodeError {
     RemoteFailure(String),
 }
 
+/// Defines the methods required to obtain a validators shuffling from a Beacon Node.
 pub trait BeaconNode: Send + Sync {
+    /// Get the shuffling for the given epoch and public key.
+    ///
+    /// Returns Ok(None) if the public key is unknown, or the shuffling for that epoch is unknown.
     fn request_shuffling(
         &self,
         epoch: u64,