docs/xl: Document pci-assignable state

docs/xl: Document pci-assignable state

Changesets 319f9a0ba9 ("passthrough: quarantine PCI devices") and
ba2ab00bbb ("IOMMU: default to always quarantining PCI devices")
introduced PCI device "quarantine" behavior, but did not document how
the pci-assignable-add and -remove functions act in regard to this.
Rectify this.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wl@xen.org>
Reviewed-by: Paul Durrant <paul@xen.org>
Release-acked-by: Juergen Gross <jgross@suse.com>