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

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

Index: native_client_sdk/src/doc/reference/sandbox_internals/index.rst
diff --git a/native_client_sdk/src/doc/reference/sandbox_internals/index.rst b/native_client_sdk/src/doc/reference/sandbox_internals/index.rst
index 3e6e9ed9eee462a2b3d89c5ae681b8d31fc87809..044da58fd1a4e0a89c57d3856dacb7adb69bacfd 100644
--- a/native_client_sdk/src/doc/reference/sandbox_internals/index.rst
+++ b/native_client_sdk/src/doc/reference/sandbox_internals/index.rst
@@ -10,9 +10,27 @@ 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>`_.