Add debugging information for nacl_io library.

native_client_sdk/src/doc/devguide/coding/nacl_io.rst

 .. _nacl_io:
 
 ###################
 The nacl_io Library
 ###################
 
 .. contents::
   :local:
   :backlinks: none
   :depth: 2
@@ skipping 194 matching lines @@
       return 2;
     }
 
     bytes_written = fwrite(data, 1, data_len, file);
 
     *output = PrintfToNewString("fwrite\1%s\1%d", file_index_string,
                                 bytes_written);
     return 0;
   }
 
-Reference information
-=====================
+Additional information
+======================
 
 The example discussed here is included in the SDK in the directory
 ``examples/demo/nacl_io_demo``.
 
 The nacl_io library is included in the SDK toolchain and is not a part of the
 Pepper API. For reference information related to the nacl_io interface see
 its header file in the SDK directory, located at
 ``include/nacl_io/nacl_io.h``.
 
+The logging system for nacl_io writes directly to the stderr stream of the NaCl
+process. For more information, see the :ref:`nacl_io section 
+<nacl-io-debugging>` of the debugging page.
+
 For more about the HTML5 file system read the `specification
 <http://dev.w3.org/2009/dap/file-system/pub/FileSystem/>`_.