[doc] [federation] Document the two types of federation better
We need some more/updated federation documentation:
- https://doc.arvados.org/master/architecture/federation.html: define the 'peer federation' and the 'login cluster federation'.
- https://doc.arvados.org/master/admin/federation.html: distinguish the 'peer federation' and the 'login cluster federation' configurations.
- Somewhere: describe the current state of groups in a login federation, particularly around user visibility and sharing.