Add debugging information for nacl_io library.

native_client_sdk/doc_generated/devguide/coding/nacl_io.html

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 <section id="the-nacl-io-library">
 <span id="nacl-io"></span><h1 id="the-nacl-io-library"><span id="nacl-io"></span>The nacl_io Library</h1>
 <div class="contents local" id="contents" style="display: none">
 <ul class="small-gap">
 <li><a class="reference internal" href="#introduction" id="id1">Introduction</a></li>
 <li><a class="reference internal" href="#using-nacl-io" id="id2">Using nacl_io</a></li>
 <li><p class="first"><a class="reference internal" href="#the-nacl-io-demo" id="id3">The nacl_io demo</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#building-and-running-the-demo" id="id4">Building and running the demo</a></li>
 <li><a class="reference internal" href="#a-look-at-the-code" id="id5">A look at the code</a></li>
 </ul>
 </li>
-<li><a class="reference internal" href="#reference-information" id="id6">Reference information</a></li>
+<li><a class="reference internal" href="#additional-information" id="id6">Additional information</a></li>
 </ul>
 
 </div><h2 id="introduction">Introduction</h2>
 <p><code>nacl_io</code> is a utility library that provides implementations of standard
 C APIs such as POSIX I/O (<code>stdio.h</code>) and BSD sockets (<code>sys/socket.h</code>).
 Its primary function is to allow code that uses these standard APIs to be
 compiled and used in a Native Client module. The library is included as part
 of Native Client SDK and is implemented in on top of Pepper API.</p>
 <p>Since Native Client modules cannot access the host machine&#8217;s file system
 directly, nacl_io provides several alternative filesystem types which
@@ skipping 170 matching lines @@
     return 2;
   }
 
   bytes_written = fwrite(data, 1, data_len, file);
 
   *output = PrintfToNewString(&quot;fwrite\1%s\1%d&quot;, file_index_string,
                               bytes_written);
   return 0;
 }
 </pre>
-<h2 id="reference-information">Reference information</h2>
+<h2 id="additional-information">Additional information</h2>
 <p>The example discussed here is included in the SDK in the directory
 <code>examples/demo/nacl_io_demo</code>.</p>
 <p>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
 <code>include/nacl_io/nacl_io.h</code>.</p>
+<p>The logging system for nacl_io writes directly to the stderr stream of the NaCl
+process. For more information, see the <a class="reference internal" href="/native-client/devguide/devcycle/debugging.html#nacl-io-debugging"><em>nacl_io section</em></a> of the debugging page.</p>
 <p>For more about the HTML5 file system read the <a class="reference external" href="http://dev.w3.org/2009/dap/file-system/pub/FileSystem/">specification</a>.</p>
 </section>
 
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