Clarify -sequenceNext:'s documentation and implementation

The documentation for `-sequenceNext:` does not mention that it ignore's the `next`'s sent from the current signal. I had thought it was a variant of `-concat:`, because I didn't look at its implementation closely enough.

After getting clarification in [#493](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/493#issuecomment-17965362), I figured the docs could use an addition mentioning that `next`'s are ignored.

Also in [#493](https://github.com/ReactiveCocoa/ReactiveCocoa/issues/493#issuecomment-17965362), it was explained that `-sequenceNext:` is a composition of `-ignoreElements`, `-concat:`, and `+defer:`. I've changed the implementation of to be built on those, which is in my eyes a simplification. Are there any performance or semantic issues to be concerned about? There are only a couple of `-sequenceNext:` tests, and they still pass.

If the doc change is acceptable but not the implementation change, I'll back it out.