NaCl SDK doc: Recover sandbox internal doc for x86-64

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 044da58fd1a4e0a89c57d3856dacb7adb69bacfd..33b5404e02507e253835f5f8884e4ea5d6b23c13 100644
--- a/native_client_sdk/src/doc/reference/sandbox_internals/index.rst
+++ b/native_client_sdk/src/doc/reference/sandbox_internals/index.rst
@@ -26,8 +26,8 @@ Native Client has sandboxes for:
   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
+* :ref:`x86-64 <x86-64-sandbox>`: 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