Document XDP device binding and RX metadata

[dylandreimerink]

This patch set (https://lore.kernel.org/all/20230119221536.3349901-2-sdf@google.com/) added the ability for XDP programs to read TX metadata from devices/drivers. It requires the XDP program to be bound to the netdev at load time (not unlike when offloading).

The following pages need to be updated:

• XDP program type - add a section with info about this
• related kfunc pages - add links back to the XDP prog type section
• libbpf and syscall pages - they currently mention prog_ifindex being only used for offloading, no longer true
• AF_XDP concept page - metadata can now be part of AF_XDP frames, update the docs to reflect this