net, xdp: Add missing xdp_features description

Add missing xdp_features field description in the struct net_device
documentation. This patch fix the following warning:

  [...]
  ./include/linux/netdevice.h:2375: warning: Function parameter or member 'xdp_features' not described in 'net_device'
  [...]

Fixes: d3d854fd

 ("netdev-genl: create a simple family for netdev stuff")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/7878544903d855b49e838c9d59f715bde0b5e63b.1675705948.git.lorenzo@kernel.org