[docs] [issue16701] Docs missing the behavior of += (in-place add) for lists.

Terry J. Reedy added the comment: Augmented assignment confuses enough people that I think we can improve the doc. In #21358 I suggest an augmented version of the previous claim, about evaluation just once. I think something here is needed perhaps even more. I have not decided what just yet. ---------- nosy: +terry.reedy versions: +Python 3.5 -Python 2.6, Python 3.2, Python 3.3 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16701> _______________________________________