Mention Result<!, E> in addition to Result<T, !> on never docs

[clarfonthey]

While the latter is mentioned as a way of saying "this computation will never fail if it returns," the former is a way of saying "this computation will never return unless it fails."

For example, it makes a lot of sense to make Command::exec return Result<!, Error>, even though it currently just returns Error (which can't be changed without breaking stability guarantees). Demonstrating both cases seems like a good idea.

[scottmcm]

I also think Result<!, E> makes a very interesting return from main.