message: add a comment to TrackedStructInDb trait
diff: @@ -15,6 +15,7 @@ pub trait TrackedStructData: InternedData {}
  impl<T: InternedData> TrackedStructData for T {}
  pub trait TrackedStructInDb<DB: ?Sized + Database>: SalsaStructInDb<DB> {
  + /// Converts the identifier for this tracked struct into a `DatabaseKeyIndex`.
  fn database_key_index(self, db: &DB) -> DatabaseKeyIndex;
  }
  