[webauthn] Editorial: use more elaborate variable names

annevk has just created a new issue for 
https://github.com/w3c/webauthn:

== Editorial: use more elaborate variable names ==
Can we just spell out what rp in rpId stands for? Or is it a widely 
understood term of art that I don't know about? Each time I see that 
I'm confused.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/359 using your GitHub account