Doc for `DoubleEndedIterator::next_back` doesn't belong on method

[It is more appropriate for the trait than the method:](https://doc.rust-lang.org/nightly/std/iter/trait.DoubleEndedIterator.html#tymethod.next_back)

> An iterator able to yield elements from both ends.
> 
> As this is the only method for this trait, the trait-level docs contain more details.
