Speaking for myself, the originally position reads better, because it doesn't introduce as many concepts/topics at the same time (i.e. I first expect to learn about building the empty elements, then I read the example and wonder about the naming, then I read the explanation below plus the example.) But in the process of writing this, you probably read over this more times than I did, so I'll defer to your for the final call. Just some input for consideration :)

By the way, "in the format expected the accessor generated by `accepts_nested_attributes_for`" is grammatically incorrect ("the format expected the"), plus it's a very clunky sentence. Would be nice if you can fix it here too!