Change return type in docstring to literal tuple to avoid bogus cross-ref. Before this change the Sphinx tried to produce cross-reference links for the returned (stdout, stderr) tuple. The docs build produced a warning because there are 2 possible cross-ref objects for `stdout`, one in `IPython.testing.iptestcontroller.TestController`, the other in `IPython.utils.capture.CapturedIO`. Neither of those is correct in the context of the `ipexec()` function. Changing literal ``(stdout, stderr)`` avoids the warning by preventing the attempt to produce cross-references.