Add Py_TPFLAGS_PREHEADER to typeobj.rst

Author: sergey-miryanov

Doc/c-api/typeobj.rst

@@ -1287,6 +1287,15 @@ and :c:data:`PyType_Type` effectively act as defaults.)
       :c:member:`~PyTypeObject.tp_weaklistoffset` field is set in a superclass.
 
 
+   .. c:macro:: Py_TPFLAGS_PREHEADER
+
+      This is a shorthand for checking is :c:macro:`Py_TPFLAGS_MANAGED_DICT` or
+      :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF` present. Supposed only for internal
+      usage.
+
+      .. versionadded:: 3.12
+
+
    .. c:macro:: Py_TPFLAGS_ITEMS_AT_END
 
       Only usable with variable-size types, i.e. ones with non-zero