PNaCl documentation: add undefined behavior

native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst

Index: native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst
diff --git a/native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst b/native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst
index d63a9439c0c0f7505ba51599a17319b510132991..18ebec7fd40616a713f6688891ae6705da5e8289 100644
--- a/native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst
+++ b/native_client_sdk/src/doc/reference/pnacl-c-cpp-language-support.rst
@@ -49,7 +49,8 @@ locations to each other as the C11/C++11 standards do.
 
 Non-atomic memory accesses may be reordered, separated, elided or fused
 according to C and C++'s memory model before the pexe is created as well
-as after its creation.
+as after its creation. Accessing atomic memory location through
+non-atomic primitives is `Undefined Behavior <undefined_behavior>`.
 
 As in C11/C++11 some atomic accesses may be implemented with locks on
 certain platforms. The ``ATOMIC_*_LOCK_FREE`` macros will always be
@@ -188,6 +189,12 @@ NaCl supports a fairly wide subset of inline assembly through GCC's
 inline assembly syntax, with the restriction that the sandboxing model
 for the target architecture has to be respected.
 
+Undefined Behavior
+==================
+
+The C and C++ languages expose some undefined behavior which is
+discussed in `PNaCl Undefined Behavior <undefined_behavior>`.
+
 Future Directions
 =================