libbpf: Extend BPF_KSYSCALL documentation

Explicitly list known quirks. Mention that socket-related syscalls can be
invoked via socketcall().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20220726134008.256968-2-iii@linux.ibm.com