VC vs credential language cleanup

[eric-schuh]

Joe's comment from last PR that is still TODO:

Overall, this is a great update. I've suggested some clean up. We also might think through how to be more rigorous in our use of credentials v certifications and certificates. There's a lot of confusion about how a real-world certification is represented in a VC and it would be good to tease out language to delineate in the use cases when it is the VC that we are talking about verses the underlying certification.