authenticator attachment terminology

[equalsJeffH]

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 #243 (comment))

[equalsJeffH]

This was fixed by merging of #309, closing manually.