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

Martin Panter added the comment: The “s += t” operation assigns the result back to s. So it could involve an extra __setattr__()/__setitem__() call, or an exception trying to modify a tuple item, etc. The extend() and slice assignment versions don’t have this extra stage. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16701> _______________________________________