libbpf: Add documentation to API functions

This adds documentation for the following API functions:

- bpf_program__set_expected_attach_type()
- bpf_program__set_type()
- bpf_program__set_attach_target()
- bpf_program__attach()
- bpf_program__pin()
- bpf_program__unpin()

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220420161226.86803-3-grantseltzer@gmail.com