What does auto std.stdio.File.ByChunkImpl byChunk (ulong chunkSize ); mean?

[kdevel]

What does

   auto std.stdio.File.ByChunkImpl byChunk (
      ulong chunkSize
   );

on https://dlang.org/library/std/stdio/file.by_chunk.html mean? 
Is that
a (forward) declaration of a function named byChunk taking a 
single
ulong argument name chunkSize. But what does that function return?