Add more text to spec about proper verification of zcap invocations #43
Description
We should add text around how verifiers should only accept zcap invocations within a certain time window from their invocation proof created date, but also highly that there are use cases for passing signed invocations through intermediaries. Verifiers that want to be even more strict can save zcap invocations for a short period of time (for replay prevention) before they fall outside of the chosen time window.