Guess channel better

[doronbehar]

Until now, when I delved into the code, I always got confused with the channels, jobsets, and the architectures. It always annoyed me how these terms are intermingled, and why e.g darwin doesn't use the unstable jobset, and what is the difference between it and master...

I also felt as if hydra-check doesn't help me a lot to hide all of these details, because whenever I wanted to check whether a package already fails on Darwin on hydra, I had to open the README, which up until now didn't include a single darwin architecture example, and try out the different jobsets...

Anyway, with the changes here, hopefully users won't be frustrated as I was :). The first 2 commits change only the docs and the --channel help text - they make the distinction between channels and jobset a bit clearer. The last commit makes the --arch choice affect the default jobset chosen, where darwin is taken into special consideration.

[makefu]

It adds the architecture to the jobset guess which is cool i think.
reasonable changes, lgtm.

[doronbehar]

Thanks!