[Tokenization] Attempt to synthesize Token Usage Type from 1 May discussion

Tokenization Task Force participants,

Yesterday we discussed [1] tokenization use cases and corresponding request data requirements
related to "token usage types”. I’ve updated the spec [2] based on the discussion and welcome
review. I’ve pasted the new text below as well. 

Ian

[1] https://www.w3.org/2018/05/01-wpwg-minutes
[2] https://w3c.github.io/webpayments-methods-tokenization/

====
Token Usage Type values

* "one-time"
   The payee expects to use the token only once (e.g., for guest checkout).

* "card-on-file"
   The payee expects to re-use the token for as yet uknown future transactions, including payer-initiated transactions and payee-initiated transactions (e.g., for partial shipment, incremental charges, and resubmission use cases). Whether and how the payee requests a new Token Cryptogram for future transactions is outside the scope of this specification.

* "recurring"
  The payee expects to re-use the token exclusively for a recurring payment according to an agreement with the payer.

--
Ian Jacobs <ij@w3.org>
https://www.w3.org/People/Jacobs/
Tel: +1 718 260 9447