Rework permissions documentation

I spent some time to improve the permissions docs. Basically, I took what @Natim had written and : 

* Removed the acronym "ACL" (it may be rather RBAC?)
* Made sure we use a consistent vocabulary
* Removed internal notions as much as possible
* Removed JSON notation when it wasn't about API

This changes highlight the following points : 

1. Permissions are hard to explain. Thus even harder to understand.
1. Permissions on ``roles`` and ``policy`` are never checked individually (thus superfluous)
1. It might be clearer if default ``role:admins`` was renamed to ``role:owners``
1. It would be wise to substitute ``system.Authenticated`` by ``role:authenticated`` and ``system.Everyone`` by ``role:everyone``