Document reparameterization race conditions on requestDelivery

It's possible for `cost_per_byte` to change between the time the user submits ` Datatrust.requestDelivery` call and the time the call is confirmed. This means that the user could end up paying an unexpectedly high price for the data. Document this issue and suggest that the user offer high gas fees to enable rapid confirmation or break up orders into multiple smaller orders to avoid issues.