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

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

-bpf_link_create()
-bpf_link_detach()
-bpf_link_update()
-bpf_link_get_next_id()
-bpf_link_get_fd_by_id()
-bpf_link_get_fd_by_id_opts()

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

Author: Jianyun Gao