docs: segmentation-offloads.txt: update for UFO depreciation

UFO is deprecated except for tuntap and packet per 0c19f846

,
("net: accept UFO datagrams from tuntap and packet"). Update UFO
docs to reflect this.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: David S. Miller <davem@davemloft.net>