libbpf: Added the description of some API functions

Currently, many API functions are not described in the document.
Add add API description of the following four API functions:
  - libbpf_set_print;
  - bpf_object__open;
  - bpf_object__load;
  - bpf_object__close.

Signed-off-by: Xin Liu <liuxin350@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221224112058.12038-1-liuxin350@huawei.com