PNaCl documentation: improve some of the SIMD documentation.

native_client_sdk/src/doc/sdk/release-notes.rst

 .. _sdk-release-notes:
 
 #############
 Release Notes
 #############
 
 Chrome/Pepper 36 (09 May 2014)
 ==============================
 
 PNaCl
 -----
 * Support `LLVM vectors
   <http://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors>`_
   and `GCC vectors
   <http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html>`_ for SIMD
-  vectors.
+  vectors through :ref:`Portable SIMD Vectors
+  <portable_simd_vectors>`. Note that this is still an early release,
+  and performance is expected to become acceptable for version 37 of
+  Chrome. More SIMD instructions will be added in later releases.
 
 Chrome/Pepper 35 (31 Mar 2014)
 ==============================
 
 PNaCl
 -----
 * Upgraded LLVM to version 3.4.
 * Translation now uses dynamic load balancing, making translation time faster.
 * Unstable pexes (i.e. non-finalized) with debug information can be loaded by
   Chrome, simplifying debugging with PNaCl. See :ref:`Debugging PNaCl pexes
@@ skipping 558 matching lines @@
 
 PPAPI
 -----
 
 * The ``CompletionCallbackFactory`` class template now takes a thread traits
   class as its second parameter. For details see the `CompletionCallbackFactory
   class template reference
   </native-client/pepper_stable/cpp/classpp_1_1_completion_callback_factory#details>`_.
 
 .. TODO: Port release notes for older releases