PNaCl documentation: add SIMD vectors.

native_client_sdk/src/doc/nacl-and-pnacl.rst

 .. _nacl-and-pnacl:
 
 ##############
 NaCl and PNaCl
 ##############
 
 This document describes the differences between **Native Client** and
 **Portable Native Client**, and provides recommendations for when to use each.
 
 .. contents::
@@ skipping 90 matching lines @@
 
 .. _when-to-use-nacl:
 
 When to use NaCl
 ================
 
 The limitations below apply to the current release of PNaCl. If any of
 these limitations are critical for your application, you should use
 non-portable NaCl:
 
-* By its nature, PNaCl does not support architecture-specific instructions in
-  an application (i.e., inline assembly). Future editions of PNaCl will
-  attempt to mitigate this problem by introducing portable intrinsics for vector
-  operations.
+* By its nature, PNaCl does not support architecture-specific
+  instructions in an application (i.e., inline assembly), but tries to
+  offer high-performance equivalents.

[Derek Schuff, 2014/04/24 16:06:20]

"high-performance portable equivalents"? 

[JF, 2014/04/24 16:16:30]

Done.

 * Currently PNaCl only supports static linking with the ``newlib``
   C standard library (the Native Client SDK provides a PNaCl port of
   ``newlib``). Dynamic linking and ``glibc`` are not yet supported.
   Work is under way to enable dynamic linking in future versions of PNaCl.
-* In the initial release, PNaCl does not support vector types and SIMD.
 * In the initial release, PNaCl does not support some GNU extensions
   like taking the address of a label for computed ``goto``, or nested
   functions.