[luatex] Undocumented callbacks

[Joseph Wright]

Hello all,

Running

    \directlua{
      for i,_ in pairs(callback.list()) do
        texio.write_nl("- " .. i)
      end
    }
    \bye

with LuaTeX 0.80.0 (TL'15) lists 58 callbacks. Comparing the list with
those in the manual, there seem to be four undocumented ones:

 - char_exists
 - find_cidmap_file
 - pdf_stream_filter_callback
 - read_cidmap_file

What is the status of these callbacks?
--
Joseph Wright