simplify hashname docs, map to JWK

[Jer Miller]

Some good questions today from @dvanduzer made me realize that the current hashname doc is pretty difficult to understand, particularly when involving references to JWK.

I'll make a pass at simplifying/minimizing it to just documenting only how it's used in telehash currently and how the existing cipher sets map to existing JWKs, not how it could be used for anything else.