Document Series' methods inherited from Promise
Refactor and document `timed`

* Make `delay()` and `timeout()` work with thenables
Fixes and documentation for `concurrent`

* `parallel()` was no longer parallel after the `Series` refactor
* `sequence()` and `parallel()` should return a `Series` instance, since they return arrays
Tweak and document `fn`

* `compose()` should assimilate thenables for consistency with other `fn` methods