Doc suggestion: show output for `next_back()` example

[xpe]

For me, at least, the example using next() and next_back() tells the story better if it shows the output.

I'm reading about the DoubleEndedIterator

Thanks to mbrubeck on #rust-beginners for these examples:

let mut iter = vec![1,2,3,4,5].into_iter(); (iter.next(), iter.next_back())
(Some(1), Some(5))

let mut i = vec![1,2,3,4,5].into_iter(); (i.next(), i.next_back(), i.next(), i.next_back())
(Some(1), Some(5), Some(2), Some(4))

I plan to come back to this issue and submit a proper documentation patch.

[GuillaumeGomez]

Nice idea!

[steveklabnik]

I would be happy to work with anyone who wants to work on this issue.

[GuillaumeGomez]

If no one does it, I can but I think it could be a nice first contribution.

[steveklabnik]

@xpe So, looking at this again, what is the issue here? I'm not sure what's significantly different here.