[docs] [issue17188] Document 'from None' in raise statement doc.

Terry J. Reedy added the comment: The double listing seems to be a glitch. Either someone else does backport or online doc change waits until I can do it. ---------- assignee: docs@python -> terry.reedy stage: patch review -> backport needed versions: +Python 3.6, Python 3.7 -Python 3.4, Python 3.5 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17188> _______________________________________