bpftool: Documentation for bpftool prog profile

Add documentation for the new bpftool prog profile command.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200309173218.2739965-3-songliubraving@fb.com