libbpf: Document bpf_{btf,link,map,prog}_get_info_by_fd()

Replace the short informal description with the proper doc comments.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230220234958.764997-1-iii@linux.ibm.com