docs: networking: Update connection tracking offload sysctl parameters

Document the following connection offload configuration parameters:
- nf_flowtable_tcp_timeout
- nf_flowtable_tcp_pickup
- nf_flowtable_udp_timeout
- nf_flowtable_udp_pickup

Signed-off-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>