tools: bpftool: add documentation for net attach/detach

Since, new sub-command 'net attach/detach' has been added for
attaching XDP program on interface,
this commit documents usage and sample output of `net attach/detach`.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>