Update documentation references to yielding arrays

 As of 2.0, the ability to yield arrays has been removed in favor of custom addYieldHandlers. See https://github.com/petkaantonov/bluebird/issues/236 and https://github.com/petkaantonov/bluebird/blob/master/changelog.md#whats-new-in-20. 

This commit updates the docs to reflect that change, and include some examples for implementing it yourself.