Sync updated documentation for bpf_redirect_map.

Sync the bpf_tcp_gen_syncookie helper function definition with the one
in tools/uapi.
Signed-off-by: NPetar Penkov <ppenkov@google.com>
Reviewed-by: NLorenz Bauer <lmb@cloudflare.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>