[webauthn] authenticator attachment terminology

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

== authenticator attachment terminology ==
wrt terminology, platform and cross_platform refer to authenticator 
(authnr) "attachment" modalities which us technologists understand and
 need when reasoning about under-the-hood-details.

I can see where the term "platform attached authenticator" or 
"platform authenticator" might work amongst users, but have concerns 
with the term "cross-platform authenticator" being confusing to users.

Perhaps we can define a "roaming authenticator" as being one with a 
"cross-platform attachment" modality. The latter term may be more 
meaningful to users, while the definition wrt attachment modality is 
meaningful to readers of the spec.

(this originally appeared as a comment on #243 here 
https://github.com/w3c/webauthn/pull/243#issuecomment-259476527)

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/308 using your GitHub account