This part of the XDP documentation is targeted at end-users, describing how to use and setup XDP.
The XDP program running (inside the driver hook point) is an eBPF program. eBPF is a general kernel facility not restricted to the XDP use-case. Thus, have its own documentation here: eBPF - extended Berkeley Packet Filter. This documentation is focused on using eBPF for the XDP specific use-case.