diff --git a/Doc/library/constants.rst b/Doc/library/constants.rst
index b02e04edaa0..80e792c9521 100644
--- a/Doc/library/constants.rst
+++ b/Doc/library/constants.rst
@@ -3,7 +3,6 @@ Built-in Constants
 
 A small number of constants live in the built-in namespace.  They are:
 
-
 .. data:: False
 
    The false value of the :class:`bool` type.
@@ -39,16 +38,23 @@ A small number of constants live in the built-in namespace.  They are:
 
    Special value used in conjunction with extended slicing syntax.
 
-   .. XXX Someone who understands extended slicing should fill in here.
-
 
 .. data:: __debug__
 
    This constant is true if Python was not started with an :option:`-O` option.
-   Assignments to :const:`__debug__` are illegal and raise a :exc:`SyntaxError`.
    See also the :keyword:`assert` statement.
 
 
+.. note::
+
+   The names :data:`None` and :data:`__debug__` cannot be reassigned
+   (assignments to them, even as an attribute name, raise :exc:`SyntaxError`),
+   so they can be considered "true" constants.
+
+   .. versionchanged:: 2.7
+      Assignments to ``__debug__`` as an attribute became illegal.
+
+
 Constants added by the :mod:`site` module
 -----------------------------------------