bpftool: Update doc (add autoattach to prog load)

Add autoattach optional to prog load|loadall for supporting
one-step load-attach-pin_link.

Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Link: https://lore.kernel.org/r/1665736275-28143-3-git-send-email-wangyufen@huawei.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>