Add migration advice about the atexit module.

pep-0290.txt

@@ -395,6 +395,21 @@ Pattern::
     "foobar"[-3:] == "bar"  -->  "foobar".endswith("bar")
 
 
+The ``atexit`` Module
+'''''''''''''''''''''
+
+The atexit module supports multiple functions to be executed upon
+program termination.  Also, it supports parameterized functions.
+Unfortunately, its implementation conflicts with the sys.exitfunc
+attribute which only supports a single exit function.  Code relying
+on sys.exitfunc may interfere with other modules (including library
+modules) that elect to use the newer and more versatile atexit module.
+
+Pattern::
+
+    sys.exitfunc = myfunc  -->  atexit.register(myfunc)
+
+
 Python 1.5 or Later
 -------------------