NaCl docs: add ARM 32-bit sandbox

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

Index: native_client_sdk/src/doc/reference/sandbox_internals/index.rst
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+.. _sandbox-internals-index:
+
+#################
+Sandbox Internals
+#################
+
+The sandbox internals explains implementation details about 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 doesn't allow developers to write platform-specific assembly).