doc: iptables provides up to 5 independent tables

This closes bugzilla:

http://bugzilla.netfilter.org/show_bug.cgi?id=807

Reported-by: Quentin Armitage <quentin@armitage.org.uk>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
doc: rpfilter: invert option should have own paragraph

Signed-off-by: Florian Westphal <fw@strlen.de>