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

Petri Lehtinen added the comment: The patch should add something to the "The from clause is used for exception..." paragraph, to state that None is also allowed. It also adds things in a confusing order. I think the new example should go below other examples, and the changedversion and addedversion directives should be the last thing in the section. ---------- nosy: +petri.lehtinen versions: -Python 3.2 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue17188> _______________________________________