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

Mariatta Wijaya added the comment: reuploaded thomir's patch issue17188_3.4.patch as is. I take no credit for this. ---------- Added file: http://bugs.python.org/file44967/issue17188_by_thomir.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17188> _______________________________________