message: storage: refine comments a bit
  Refine documentation related to storage subsystem, there should be
  no functional changes.
diff: @@ -316,18 +316,16 @@ impl BlobCache for FileCacheEntry {
  let mut total_size = 0;
  let mut start = 0;
  while start < pending.len() {
  + // Be careful that `end` is inclusive.
  let mut end = start;
  - // `end` should be inclusive within `pending` slice.
  + // Figure out the range with continuous chunk ids.
  while end < pending.len() - 1 && pending[end + 1].id() == pending[end].id() + 1 {
  end += 1;
  }
  - // Find a range with continuous chunk id
  + // Don't forget to clear its pending state whenever backend IO fails.
  let blob_offset = pending[start].compressed_offset();
  -
  - // NOTE: Please be aware that being fetched chunks include index=`end` chunk in below `pending` slice.
  - // Do't forget to clear its pending state whenever backend IO fails.
  let blob_end = pending[end].compressed_offset() + pending[end].compressed_size() as u64;
  let blob_size = (blob_end - blob_offset) as usize;
  @@ -349,7 +347,7 @@ impl BlobCache for FileCacheEntry {
  }
  }
  Err(_e) => {
  - // NOTE: We must clear pending flag for `end` chunk.
  + // Clear the pending flag for all chunks in processing.
  for chunk in &mut pending[start..=end] {
  self.chunk_map.clear_pending(chunk);
  }
  