Test propagation of additional downstream result types

[basil]

#24 changed the behavior of this plugin to propagate the result of downstream builds. However, only propagation of unstable builds was covered in unit tests, and the documentation was not updated. This PR adds test coverage for the other types of downstream statuses and updates the documentation.

[basil]

FYI, I think #24 is going in the right direction, but it also makes me a bit nervous, because I'm not sure all of the parallel steps in my pipelines are going to correctly deal with the downgrade of build results as a result of this change in behavior. I may find some regressions in my jobs and have to introduce propagate: false and propagate the results from parallel steps manually instead. I expressed my use case in more detail in JENKINS-49073 (comment). As I described in that comment, I think allowing the parallel step to propagate the "worst" result from a set of parallel branches would be very helpful.

[basil]

@dwnusbaum Gentle ping. No rush on this, but just wanted to make sure the first notification didn't slip by.

[dwnusbaum]

@basil Sorry, it didn't slip by, I've just been swamped lately with other work. Not sure when I'll be able to take a look through open PRs.

[dwnusbaum]

Thanks!