CREDENTIAL: expanding public suffix scope

[annevk]

Did @sleevi et al agree with expanding the scope of public suffixes?

We should probably add some warnings around these algorithms so they are not picked up by other specifications if we really want to go down this route. And make it somewhat clearer public suffix is involved as does origin A match origin B does not make it really clear.

[mikewest]

I haven't talked with @sleevi about it, no. Happy to change things if they end up not making sense.