Use "impl-detail" directive where applicable.
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index 1762ddc..72f292d 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -8,7 +8,7 @@
 The CPython interpreter scans the command line and the environment for various
 settings.
 
-.. note::
+.. impl-detail::
 
    Other implementations' command line schemes may differ.  See
    :ref:`implementations` for further resources.