#9451: strengthen warning about __*__ special name usage.

Doc/reference/lexical_analysis.rst

@@ -362,11 +362,12 @@ characters:
       information on this convention.
 
 ``__*__``
-   System-defined names.  These names are defined by the interpreter and its
-   implementation (including the standard library); applications should not expect
-   to define additional names using this convention.  The set of names of this
-   class defined by Python may be extended in future versions. See section
-   :ref:`specialnames`.
+   System-defined names. These names are defined by the interpreter and its
+   implementation (including the standard library).  Current system names are
+   discussed in the :ref:`specialnames` section and elsewhere.  More will likely
+   be defined in future versions of Python.  *Any* use of ``__*__`` names, in
+   any context, that does not follow explicitly documented use, is subject to
+   breakage without warning.
 
 ``__*``
    Class-private names.  Names in this category, when used within the context of a