NaCl documentation: add links for x86-{32,64} and MIPS32.

native_client_sdk/src/doc/reference/sandbox_internals/index.rst

 .. _sandbox-internals-index:
 
 #################
 Sandbox Internals
 #################
 
 The sandbox internals documentation describes implementation details for
 Native Client sandboxing, which is also used by Portable Native
 Client. These details can be useful to reimplement a sandbox, or to
 write assembly code that follows sandboxing rules for Native Client
 (Portable Native Client does not allow platform-specific assembly code).
 
+As an implementation detail, the Native Client sandboxes described here
+are currently used by Portable Native Client to execute code on the
+corresponding machines in a safe manner. The portable bitcode contained
+in a **pexe** is translated to a machine-specific **nexe** before
+execution. This may change at a point in time: Portable Native Client
+doesn't necessarily need these sandboxes to execute code on these
+machines. Note that the Portable Native Client compiler itself is also
+untrusted: it too runs in a Native Client sandbox described below.
+
 Native Client has sandboxes for:
 
-* :ref:`ARM 32-bit<arm-32-bit-sandbox>`.
-* x86-32
-* x86-64
-* MIPS32
+* :ref:`ARM 32-bit <arm-32-bit-sandbox>`.
+* x86-32: the original design is described in `Native Client: A Sandbox
+  for Portable, Untrusted x86 Native Code
+  <http://research.google.com/pubs/archive/34913.pdf>`_, the current
+  design has changed slightly since then.
+* x86-64: the original design is described in `Adapting Software Fault
+  Isolation to Contemporary CPU Architectures
+  <http://research.google.com/pubs/archive/35649.pdf>`_, the current
+  design has changed slightly since then.
+* MIPS32, described in the `overview of Native Client for MIPS
+  <https://code.google.com/p/nativeclient/issues/attachmentText?id=2275&aid=22750018000&name=native-client-mips-0.4.txt>`_,
+  and `bug 2275
+  <https://code.google.com/p/nativeclient/issues/detail?id=2275>`_.