User restriction updates

From Kenny: 

 
Hey, I made some change to the superuser feature in https://github.com/quay/quay/pull/1551. Once it gets merge, I think we’ll need a few docs changes: * RESTRICTED_USER_READ_ONLY is no longer valid, so it can be removed

• FEATURE_RESTRICTED_USERS : With this feature enabled, restricted users cannot create organizations, or content in their own namespace. The normal permissions applies for organizations membership. i.e A restricted user will still have normal permission in organizations based on the teams they are members of.