Update docs and screen shots for Coupons

- percentage must be an integer (or it will be silently truncated)
- default behavior is to delay the coupon until payment is due, and there is no longer an option
- edit, re-organize and add screen shots
@wsmoak Do you know what this even means? "Coupons will appear on a customer's record in the order of "charge, adjustment, payment".  I think we should clarify this or remove it. I know all you did was reformat it :)