Add missing documentation for conntrack, snat_reroute and sync_version.

Also fix up a typo, IPVS_DEBUG should be IP_VS_DEBUG.
Acked-by: NJulian Anastasov <ja@ssi.bg>
Acked-by Hans Schillstrom <hans@schillstrom.com>
Signed-off-by: NSimon Horman <horms@verge.net.au>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>