tools/bpftool: Add documentation and sample output for process info

Add statements about bpftool being able to discover process info, holding
reference to BPF map, prog, link, or BTF. Show example output as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-10-andriin@fb.com