libbpf: Add documentation to map pinning API functions

This adds documentation for the following API functions:

- bpf_map__set_pin_path()
- bpf_map__pin_path()
- bpf_map__is_pinned()
- bpf_map__pin()
- bpf_map__unpin()
- bpf_object__pin_maps()
- bpf_object__unpin_maps()

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/20230126024225.520685-1-grantseltzer@gmail.com