bpf: Document BPF_PROG_ATTACH syscall command

Document the prog attach command in more detail, based on git commits:
* commit f4324551 ("bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH
  commands")
* commit 4f738adb ("bpf: create tcp_bpf_ulp allowing BPF to monitor
  socket TX/RX data")
* commit f4364dcf ("media: rc: introduce BPF_PROG_LIRC_MODE2")
* commit d58e468b ("flow_dissector: implements flow dissector BPF
  hook")

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-6-joe@cilium.io