libbpf: Add doxygen documentation for btf/iter etc. in bpf.h

Add doxygen comment blocks for emaining helpers (btf/iter etc.) in
tools/lib/bpf/bpf.h. These doc comments are for:

-libbpf_set_memlock_rlim()
-bpf_btf_load()
-bpf_iter_create()
-bpf_btf_get_next_id()
-bpf_btf_get_fd_by_id()
-bpf_btf_get_fd_by_id_opts()
-bpf_raw_tracepoint_open_opts()
-bpf_raw_tracepoint_open()
-bpf_task_fd_query()

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

Author: Jianyun Gao