Only create promo items if order is eligible

Add a doc block to CreateLineItem.perform block to explain why it's not
suitable for Add to Cart events and explained another possible caveat

Stores running with this action set need to make sure the promo event is Order Contents Changed instead of Add to Cart. Otherwise they might not get the expected result. Will look into an update on spree-guides about this.

ci is green on master. Will run it on 2-0-stable now as I believe we need it there too.