We're in the process of overhauling the RBAC documentation in keystone
The token-overview.rst doc contains some nice context about
authorization scopes and it's already in the administrator guide.
This commit updates the details about each scope and try to make it a
little easier to read.
I plan on linking to this document from the new RBAC document.
Change-Id: Id2ad40947135fef73005d5f03d28882b68638579