NaCl documentation: improve building documentation

native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst

 ============================
 PNaCl C/C++ Language Support
 ============================
 
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 Source language support
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 Communication between threads should use atomic primitives as described
 in `Memory Model and Atomics`_.
 
 ``setjmp`` and ``longjmp``
 ==========================
 
 PNaCl and NaCl support ``setjmp`` and ``longjmp`` without any
 restrictions beyond C's.
 
+.. _exception_handling:
+
 C++ Exception Handling
 ======================
 
 PNaCl currently supports C++ exception handling through ``setjmp()`` and
 ``longjmp()``, which can be enabled with the ``--pnacl-exceptions=sjlj``
 linker flag. Exceptions are disabled by default so that faster and
 smaller code is generated, and ``throw`` statements are replaced with
 calls to ``abort()``. The usual ``-fno-exceptions`` flag is also
 supported. PNaCl will support full zero-cost exception handling in the
 future.
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   A similar feature is **thread suspension**: The ability to
   asynchronously suspend and resume a thread and inspect or modify its
   execution state (such as register state).
 
   Neither PNaCl nor NaCl currently support asynchronous interruption
   or suspension of threads.
 
 If PNaCl were to support either of these, the interaction of
 ``volatile`` and atomics with same-thread signal handling would need
 to be carefully detailed.