Correct behavior of `after` and `before`

In case you're checking my work, this behavior is not directly documented nor is is implemented by jQuery 1.x. I've opened https://github.com/jquery/api.jquery.com/pull/580 to fix the documentation--we should probably hold off on merging this until someone on the jQuery team can confirm.

Commit message:

> According to the jQuery API documentation, each of these methods should
> be invoked with a second argument (mimicking the behavior of similar
> signatures for `append` and `prepend`).