Clarify iterator protocol and .next_back()

The iterator protocol documents that behavior on further calls after `.next()` returns None is open to decide for the implementation. We should clarify in `DoubleEndedIterator` that `.next_back()` is to be understood in the same way, and that calls to `next` and `next_back` are interchangeable — any of the two may return `None`, and after that point, the iteration is done the same way.
