Document `before` and `after` behavior

[schneems]

First of all I love your project. Thank you for your work. I hit a bit of an issue with some of my expectations of this library and was wondering if we can clarify what should be expected to happen with different before and after blocks with this gem.

Here's my raw issue heroku/hatchet#110. In the issue we're basically running a test in the background via a thread, and then joining the threads in anafter task. It looks like when a test fails due to an after task that it does not get re-run. Is that correct? Can we document the behavior? Would it be easy/possible to re-run the whole test that fails in this way?

I'm happy to send you a doc PR, but I want to make sure I understand the desired behavior here first.