gh-67056: document that registering/unregistering an atexit func from within an atexit func is undefined

Doc/library/atexit.rst

@@ -20,6 +20,9 @@ at interpreter termination time they will be run in the order ``C``, ``B``,
 program is killed by a signal not handled by Python, when a Python fatal
 internal error is detected, or when :func:`os._exit` is called.
 
+**Note:** The effect of registering or unregistering functions from within
+a cleanup function is underfined.
+
 .. versionchanged:: 3.7
     When used with C-API subinterpreters, registered functions
     are local to the interpreter they were registered in.

Misc/NEWS.d/next/Documentation/2023-05-14-12-11-28.gh-issue-67056.nVC2Rf.rst

@@ -0,0 +1,2 @@
+Document that the effect of registering or unregistering an :mod:`atexit`
+cleanup function from within a registered cleanup function is undefined.