Document that sys.tracebacklimit can be set to None

Add `versionchanged` marker

Closes: python#127555

Author: srittau

Doc/library/sys.rst

@@ -1968,8 +1968,11 @@ always available.
    When this variable is set to an integer value, it determines the maximum number
    of levels of traceback information printed when an unhandled exception occurs.
    The default is ``1000``.  When set to ``0`` or less, all traceback information
-   is suppressed and only the exception type and value are printed.
+   is suppressed and only the exception type and value are printed.  Setting it
+   to ``None`` resets the limit to the default value.
 
+   .. versionchanged:: 3.6.4
+      Can now be set to ``0`` or ``None``.
 
 .. function:: unraisablehook(unraisable, /)