extensions: libxt_cluster: add note on arptables-jf

Gao feng reported problems while getting the cluster match working with
arptables. This patch adds a note in the manpage to warn about the arptables-jf
syntax, which is different from mainstream arptables.

Reported-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>