Mention the new bit in the GC header

pep-0442.txt

@@ -82,7 +82,7 @@ Impact
 While this PEP discusses CPython-specific implementation details, the
 change in finalization semantics is expected to affect the Python
 ecosystem as a whole.  In particular, this PEP obsoletes the current
-guideline that "objects with a __del__ method should not be part of a
+guideline that "objects with a ``__del__`` method should not be part of a
 reference cycle".
 
 
@@ -170,6 +170,11 @@ they are not finalized and end up in ``gc.garbage``.  However, a non-NULL
 ``tp_del`` is not encountered anymore in the CPython source tree (except
 for testing purposes).
 
+On the internal side, a bit is reserved in the GC header for GC-managed
+objects to signal that they were finalized.  This helps avoid finalizing
+an object twice (and, especially, finalizing a CT object after it was broken
+by the GC).
+
 
 Discussion
 ==========