ipvs: Add documentation for new sysctl entries

Add missing documentation for conntrack, snat_reroute and sync_version.

Also fix up a typo, IPVS_DEBUG should be IP_VS_DEBUG.

Acked-by: Julian Anastasov <ja@ssi.bg>
Acked-by Hans Schillstrom <hans@schillstrom.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>