[NaCl SDK] docs: Introduce .bc vs .pexe naming convention before using that in doc.

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

 .. _devcycle-debugging:
 
 #########
 Debugging
 #########
 
 This document describes tools and techniques you can use to debug, monitor,
 and measure your application's performance.
 
 .. contents:: Table Of Contents
@@ skipping 204 matching lines @@
 
 .. _debugging_pnacl_pexes:
 
 Debugging PNaCl pexes (with Pepper 35+)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 If you want to use GDB to debug a program that is compiled with the PNaCl
 toolchain, you must have a copy of the pexe from **before** running
 ``pnacl-finalize``. The ``pnacl-finalize`` tool converts LLVM bitcode
 to the stable PNaCl bitcode format, but it also strips out debug
-metadata, which we need for debugging.
+metadata, which we need for debugging. In this section we'll give the
+LLVM bitcode file a ``.bc`` file extension, and the PNaCl bitcode file
+a ``.pexe`` file extension. The actual extension should not matter, but
+it helps distinguish between the two types of files.
 
 **Note** unlike the finalized copy of the pexe, the non-finalized debug copy
 is not considered stable. This means that a debug copy of the PNaCl
 application created by a Pepper N SDK is only guaranteed to run
 with a matching Chrome version N. If the version of the debug bitcode pexe
 does not match that of Chrome then the translation process may fail, and
 you will see and error message in the JavaScript console.
 
 Also, make sure you are passing the ``-g`` :ref:`compile option
 <compile_flags>` to ``pnacl-clang`` to enable generating debugging info.
@@ skipping 352 matching lines @@
 <http://www.chromium.org/nativeclient>`_ that describe how to do profiling on
 `64-bit Windows
 <https://sites.google.com/a/chromium.org/dev/nativeclient/how-tos/profiling-nacl-apps-on-64-bit-windows>`_
 and `Linux
 <http://www.chromium.org/nativeclient/how-tos/limited-profiling-with-oprofile-on-x86-64>`_
 machines.
 
 
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