message: Add clarifying comment to datasize skip on `serialized_item_pool`
diff: @@ -201,6 +201,7 @@ pub struct StorageInner {
  /// An in-memory pool of already loaded serialized items.
  ///
  /// Keyed by serialized item ID, contains the serialized item.
  + // Note: `DataSize` is skipped here to avoid incurring locking overhead.
  #[data_size(skip)]
  serialized_item_pool: RwLock<ObjectPool<Box<[u8]>>>,
  /// The fraction of validators, by weight, that have to sign a block to prove its finality.
  