RBAC documentation talks about groups instead of roles

[lubosmj]

https://pulpproject.org/pulp_container/docs/admin/learn/rbac/#distribution-collaborators

We should discuss roles only, like it is done in pulp-file: https://pulpproject.org/pulp_file/docs/admin/guides/01-rbac/#creator-role.

[lubosmj]

We should also add a note about the private flag with regard to disabled token authentication at Role-based Access Control - Pulp Project. It is not very transparent how we document the behaviour. We can once again mention in this section that when token authentication is disabled, the private flag is ignored.

Then, we should highlight in bold the note about the private flag at Authentication.