There is no need for OAuth documentation. OAuth is a standard protocol, which has own documentation, RFC (you already have that Oauth token_url - #2 by jangaraj - each endpoint has documented request/response in the RFC).
I guess you have wrong expectation - OAuth is not for “I have my own API so I would use it as OAuth in the Grafana”, but it is for “I have OAuth compliant Identity provider server/service, so I use it as OAuth in the Grafana”. Apparently you don’t have OAuth service, so that is a reason why you have a problem.