message: Add a comment to `requests_sent` field.
diff: @@ -170,6 +170,8 @@ where
  /// Keeps track of the lowest/oldest seen unit per validator when syncing.
  /// Used only for logging.
  oldest_seen_panorama: ValidatorMap<Option<u64>>,
  + /// Keeps track of the requests we've sent so far and the recipients.
  + /// Used to decide whether we should ask more nodes for a particular dependency.
  requests_sent: BTreeMap<Dependency<C>, HashSet<I>>,
  /// Boolean flag indicating whether we're synchronizing current era.
  pub(crate) current_era: bool,
  