libbpf: Add doxygen documentation for bpf_obj_* APIs in bpf.h

Add doxygen comment blocks for all public bpf_obj_* APIs in
tools/lib/bpf/bpf.h. These doc comments are for:

-bpf_obj_pin()
-bpf_obj_pin_opts()
-bpf_obj_get()
-bpf_obj_get_opts()
-bpf_obj_get_info_by_fd()

Signed-off-by: Jianyun Gao <[email protected]>

Author: Jianyun Gao