doc: add notes on ENA usage on metal instances

doc: add notes on ENA usage on metal instances

As AWS metal instances are supporting IOMMU, the usage of igb_uio or
vfio-pci can lead to a problems (when to use which module), especially
that the vfio-pci isn't supporting SMMU on arm64.

To clear up the problem of using those modules in various setup
conditions (with or without IOMMU) on metal instances, more detailed
explanation was added.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>