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

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

-bpf_map_create()
-bpf_map_update_elem()
-bpf_map_lookup_elem()
-bpf_map_lookup_elem_flags()
-bpf_map_lookup_and_delete_elem()
-bpf_map_lookup_and_delete_elem_flags()
-bpf_map_delete_elem()
-bpf_map_delete_elem_flags()
-bpf_map_get_next_key()
-bpf_map_freeze()
-bpf_map_get_next_id()
-bpf_map_get_fd_by_id()
-bpf_map_get_fd_by_id_opts()

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

Author: Jianyun Gao