message: add comment about into_dirty_writes
diff: @@ -52,6 +52,8 @@ impl Cache {
  self.cached_values.get(key).map(|(_dirty, value)| value)
  }
  + /// Consumes self and returns only written values as values that were only read must be filtered
  + /// out to prevent unecessary writes.
  fn into_dirty_writes(self) -> HashMap<Key, StoredValue> {
  self.cached_values
  .into_iter()
  