doc: clarify IOMMU disabling for uio_pci_generic

doc: clarify IOMMU disabling for uio_pci_generic

If the devices used for DPDK are bound to the "uio_pci_generic" kernel
module, the IOMMU should be disabled in order not to break the IO
transmission because of the virtual / physical address mapping.

The patch clarifies the IOMMU configurations on both x86_64 and arm64
systems.

Signed-off-by: Tone Zhang <tone.zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Rami Rosen <roszenrami@gmail.com>
Acked-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>