tools/bpftool: Add documentation and bash-completion for `link detach`

Add info on link detach sub-command to man page. Add detach to bash-completion
as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com.>
Link: https://lore.kernel.org/bpf/20200731182830.286260-6-andriin@fb.com