documentation/bpf: Document cgroup unix socket address hooks

Update the documentation to mention the new cgroup unix sockaddr
hooks.

Signed-off-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Link: https://lore.kernel.org/r/20231011185113.140426-8-daan.j.demeyer@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>