Use "impl-detail" directive where applicable.
diff --git a/Doc/reference/executionmodel.rst b/Doc/reference/executionmodel.rst
index 57a2205..9f6170d 100644
--- a/Doc/reference/executionmodel.rst
+++ b/Doc/reference/executionmodel.rst
@@ -128,7 +128,7 @@
 itself.  ``__builtins__`` can be set to a user-created dictionary to create a
 weak form of restricted execution.
 
-.. note::
+.. impl-detail::
 
    Users should not touch ``__builtins__``; it is strictly an implementation
    detail.  Users wanting to override values in the built-in namespace should