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

Nitika Agarwal added the comment: Hi, As suggested by Petri,I have made some changes. Please review the attached patch.Any comments and feedback are welcome. ---------- nosy: +nitika Added file: http://bugs.python.org/file34431/issue17188.patch _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17188> _______________________________________