document SandboxedNativeEnvironment pattern (#2063)

Author: davidism

docs/nativetypes.rst

@@ -55,6 +55,17 @@ Foo
 >>> print(result.value)
 15
 
+Sandboxed Native Environment
+----------------------------
+
+You can combine :class:`.SandboxedEnvironment` and :class:`NativeEnvironment` to
+get both behaviors.
+
+.. code-block:: python
+
+    class SandboxedNativeEnvironment(SandboxedEnvironment, NativeEnvironment):
+        pass
+
 API
 ---