Add debugging information for nacl_io library.

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

Index: native_client_sdk/src/doc/devguide/coding/nacl_io.rst
diff --git a/native_client_sdk/src/doc/devguide/coding/nacl_io.rst b/native_client_sdk/src/doc/devguide/coding/nacl_io.rst
index 7d5b721ffb471e8f39333d01898aec0f847bece0..a6b6ed59ae8c6ddc9e5402c66260bae2d3c8667f 100644
--- a/native_client_sdk/src/doc/devguide/coding/nacl_io.rst
+++ b/native_client_sdk/src/doc/devguide/coding/nacl_io.rst
@@ -212,8 +212,8 @@ below. Notice that it does not contain any PPAPI calls and looks like
     return 0;
   }
 
-Reference information
-=====================
+Additional information
+======================
 
 The example discussed here is included in the SDK in the directory
 ``examples/demo/nacl_io_demo``.
@@ -223,5 +223,9 @@ 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/>`_.