[doc] Additional information to configure authorization and default roles

Important Box mention that the user name 'admin' is used as role, same applies to the group.

As result a user X member of group 'admin' will get the role admin and its permissions.

If the default roles are not wanted, they can not be removed, the permission can be set explicitly with the <role name="admin" permission="..."/>.
To disable the role the permission can be set to "NONE", note that empty permission will cause Exceptions.