explain "registration" earlier

[samuelweiler]

1.1.2 seems to assume the reader understands what registration is. The Note in section 1.2.2 might be usefully moved up to 1.1.2 and/or 1.2.1, to provide some context earlier.

(Note: The cross-origin use of SPC credentials makes it possible for the user to "register once" and authenticate on any merchant origin, not just the merchant origin where the user first registered for SPC.)

[stephenmcgruer]

@ianbjacobs - can I ask you to own this as its part of the non-normative Introduction section?

[ianbjacobs]

@samuelweiler, you indicated that pull request #146 may not be sufficient to close this issue. We do have an explainer:
https://github.com/w3c/secure-payment-confirmation/blob/main/explainer.md

Would it make sense to refer the reader to the explainer in the introduction for more background? And would doing so satisfy your concern in this issue?

[samuelweiler]

The second commit makes a big difference. I'm fine without a cite to the explainer.