Update language regarding idempotency

Small change, after a conversation with @mezis, to clarify GET, POST, PATCH idempotency.

This still raises a question in my mind about how the state of an entity for a particular hypermedia URL. Take, for example,`https://bookings.example.com/api/bookings/123`. The `123` in that URL implies an ID but, if the state of the object is fixed for each hypermedia URL in a response, then that is presumably some form of timestamp, or reference to an object at a particular moment in time, correct?

Also, how do we intend to manage storing the states across time? Is that an implementation detail for each service, or are we standardising on something like the audit gem [Papertrail](https://github.com/airblade/paper_trail), or (quite likely) have I misunderstood?