Document useFetch

No documentation for this option: 
<img width="393" alt="Screen Shot 2021-04-20 at 3 56 04 PM" src="https://user-images.githubusercontent.com/490968/115473642-ea4bef00-a1f0-11eb-8230-aac6250c277f.png">

https://api.emberjs.com/ember-data/release/classes/RESTAdapter/properties/useFetch?anchor=useFetch