Documentation lacks custom OmniAuth provider details

I want to use a custom OmniAuth provider, but the documentation is missing some things. Mainly, what user attributes can a provider give? I have a provider that could provide name, email, username, groups, and whether or not the user is an admin. There is no list of available attributes.

I am going to use omniauth-oauth2 with my own provider which provides all of the information above and possibly more in the future (such as website, bio, etc.). It also provides the user's image as a base64 encoded PNG, but I could also use a custom Gravatar server as that is officially supported.