Mention pitfails of c_div() implementation

Author: skirpichev

peps/pep-0791.rst

@@ -81,8 +81,8 @@ and issue `python/cpython#81313
 * ``c_div()`` and ``n_div()`` --- for integer division with rounding towards
   positive infinity (ceiling divide) and to the nearest integer, see `relevant
   discussion thread <https://discuss.python.org/t/91269>`_.  This is reinvented
-  several times in the stdlib, e.g. in :mod:`datetime` and
-  :mod:`fractions`.
+  several times in the stdlib, e.g. in :mod:`datetime` and :mod:`fractions`.
+  And it's easy to do this wrongly, as demonstrated by the thread.
 * ``gcdext()`` --- to solve linear `Diophantine equation
   <https://en.wikipedia.org/wiki/Diophantine_equation>`_ in two variables (the
   :external+py3.14:class:`int` implementation actually includes an extended