netfilter: doc: add nf_conntrack sysctl api documentation

I grepped through the code and picked bits about nf_conntrack sysctl api
and put that into one documentation file.

[ I have mangled this patch including comments from several grammar
  improvements proposed by Neal Murphy <neal.p.murphy@alum.wpi.edu>,
  any new grammar error is my mistake --pablo ]

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>