Yes, the API doc is out of date.
To get an oauth2 token you can follow How can I get an oauth2 token?
curl -X POST -d 'grant_type=password&username=${username}&password=${password}&client_id=${client_id}&client_secret=${client_secret}' https://online-go.com/oauth2/token/
If you already have a token and it expired, you can get a new one with
curl -X POST -d 'grant_type=refresh_token&username=${username}&refresh_token=${refresh_token}&client_id=${client_id}&client_secret=${client_secret}' https://online-go.com/oauth2/token/
If you got the token you can add it to the header of your requests as Authorization=Bearer ${auth_token}