Add a new directive marking up implementation details and start using it.
diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
index d23e53a..96036e9 100644
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -1,4 +1,3 @@
-
:mod:`weakref` --- Weak references
==================================
@@ -76,9 +75,10 @@
obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
-Other built-in types such as :class:`tuple` and :class:`long` do not support
-weak references even when subclassed (this is an implementation detail and may
-be different across various Python implementations).
+.. impl-detail::
+
+ Other built-in types such as :class:`tuple` and :class:`long` do not support
+ weak references even when subclassed.
Extension types can easily be made to support weak references; see
:ref:`weakref-support`.