Add debugging information for nacl_io library.

native_client_sdk/src/doc/devguide/devcycle/debugging.rst

Index: native_client_sdk/src/doc/devguide/devcycle/debugging.rst
diff --git a/native_client_sdk/src/doc/devguide/devcycle/debugging.rst b/native_client_sdk/src/doc/devguide/devcycle/debugging.rst
index b00adb6526514554e358884825fedc450dda0902..0c3b660e21f6516066170fdd033adccc2621ddb0 100644
--- a/native_client_sdk/src/doc/devguide/devcycle/debugging.rst
+++ b/native_client_sdk/src/doc/devguide/devcycle/debugging.rst
@@ -89,6 +89,7 @@ calling fprintf() directly, or by using cover functions like these:
 By default stdout and stderr will appear in Chrome's stdout and stderr stream
 but they can also be redirected as described below.
 
+.. _redirecting-output-to-log:
 
 Redirecting output to log files
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -513,8 +514,29 @@ to just their first letter (``b`` for break, ``c`` for continue, and so on).
 To interrupt execution of your module, press <Ctrl-c>. When you're done
 debugging, close the Chrome window and type ``q`` to quit gdb.
 
-Debugging with other tools
-==========================
+Non-Standard Debugging
+======================
+
+.. _nacl-io-debugging:
+
+nacl_io
+-------
+
+The logging system for the :doc:`nacl_io library <../coding/nacl_io>` writes
+directly to the ``stderr`` stream of the NaCl process. It deliberately bypasses
+the standard library functions implemented in nacl_io to avoid circular calls to
+itself.

[Sam Clegg, 2014/09/30 17:17:02]

I would put this paragraph in the NaCl document rather than the debugging
document.  Its basically saying that while nacl_io redirects things like
printf() the redirection doesn't apply to its own internal logging.  The
internal logging appears as normal stderr as described in this page already.  

So basically what this paragraph says is that nacl_io logging does go to the
standard place, but most other I/O under nacl_io does not.  Sorry if thats not a
very clear statement :(

[Sam Clegg, 2014/09/30 17:28:37]

Sorry, I meant to write "the nacl_io documentation".

[jpmedley, 2014/10/01 21:17:11]

Done.

+
+The stderr from the process is by default visible in the terminal that launched
+chrome (at least on mac and linux). Be sure that when you launch chrome it
+doesn't attach to an existing instance. One simple way to do this is to pass a
+new directory to chrome as your user data directory (``chrome
+--user-data-dir=<newdir>``). It's also possible to redirect the NaCl's stderr to
+other places such as, for example, :ref:`log files  
+<redirecting-output-to-log>`. 

[Sam Clegg, 2014/09/30 17:17:02]

This second paragraph is just about how to capture stderr of chrome and is not
related to nacl_io in particular.  If you want to keep the detail about
--user-data-dir you could add it above to the paragraph that reads: "By default
stdout and stderr will appear in Chrome's stdout and stderr stream..."

[jpmedley, 2014/10/01 21:17:11]

Done.

+
+Alternative Debuggers
+---------------------
 
 If you cannot use the :ref:`Visual Studio add-in <visual_studio>`, or you want
 to use a debugger other than nacl-gdb, you must manually build your module as a