[docs] [issue18573] In unittest.TestCase.assertWarns doc there is some text about assertRaises()

Changes by Terry J. Reedy <tjreedy@udel.edu>: ---------- stage: -> needs patch versions: +Python 2.7, Python 3.3 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue18573> _______________________________________