"Authenticator argument" vs. "authenticator data value" in Extensions section

[vijaybh]

It seems like we sometimes use one of these terms and sometimes the other. We should clean up the wording here.

[selfissued]

The defined terms [=client extension input=], [=client extension output=], [=authenticator extension input=], and [=authenticator extension output=] are now used in place of the previously inconsistent terminology that the issue called out. These were added by PR #389 .