message: Add comment about block_len
diff: @@ -14,6 +14,8 @@ const EMPTY_DATA: [u8; 0] = [0u8; 0];
  /// Positions on the other hand, are optionally entirely decoded upfront.
  pub struct SegmentPostings<'a> {
  len: usize,
  + // Removing this makes the code slower
  + // See https://github.com/tantivy-search/tantivy/issues/89
  block_len: usize,
  doc_offset: u32,
  block_decoder: BlockDecoder,
  