Add missing tokens for payment transactions

I made similar consistency changes to the other tokens by changing $payment to $transaction. I'm actually not sure what the compelling reason was for naming this variable $transaction instead of $payment... other than a "payment transaction" could technically be a refund in addition to a payment.

I also removed instance_id and payload from the list, as these are really just internal variables having to do with a) the Rule used to define a payment method's settings and b) the record of communication between the payment service and the site.