doc: mention SNAT in INPUT chain since kernel 2.6.36

SNAT in the INPUT chain was added Jun 2010 to the kernel
(commit c68cd6cc21eb329c47ff020ff7412bf58176984e).

Signed-off-by: Michael Roth <mail@mroth.net>
Signed-off-by: Florian Westphal <fw@strlen.de>