[doc] Add initial system-scoped text to secure-rbac
[doc] Add initial system-scoped text to secure-rbac

I never got around to adding in an initial pass on system scoped
interaction and use with secure rbac. This change adds a high level
overview to help explain the context.

Change-Id: I4dca32c882f484e75378aca8bb043ebd078a13cf

openstack/ironic,openstack/ironic