Update allowed access docs to take account of recently added properties

[manics]

Proposed change

OAuthenticator has had some significant changes affecting how access can be configured, including

• Add allowed_scopes to all authenticators to allow some users based on granted scopes #719
• [All] Move group management from generic to base oauthenticator (allowed_groups, admin_groups, auth_state_groups_key) #735

https://oauthenticator.readthedocs.io/en/latest/topic/allowing.html is missing these new properties.

Alternative options

Do nothing, rely on users reading the Traitlets API documentation

Who would use this feature?

JupyterHub admins

(Optional): Suggest a solution

Update https://github.com/jupyterhub/oauthenticator/blob/main/docs/source/topic/allowing.md