Changes to QA

Hi @scopatz - would you mind reviewing this PR? I just ask because it's entirely focused on the section you did for QA. It's mostly spelling and style, but there are a few more major things. In particular, I added a diagram that might be helpful.

Let me know if there's anything you don't agree with. Not much of it has substance except the new diagram (that will come later... I just struggle to read the content of text while spelling and grammar issues are still at large).

Thanks!