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``
Thanks for taking the time to update this. However, it seems that we are moving away from this approach now. e.g. when the refactoring of the ACLs will be done, we'll not have any roles anymore.