docs: nf_flowtable: update documentation with enhancements

This patch updates the flowtable documentation to describe recent
enhancements:

- Offload action is available after the first packets go through the
  classic forwarding path.
- IPv4 and IPv6 are supported. Only TCP and UDP layer 4 are supported at
  this stage.
- Tuple has been augmented to track VLAN id and PPPoE session id.
- Bridge and IP forwarding integration, including bridge VLAN filtering
  support.
- Hardware offload support.
- Describe the [OFFLOAD] and [HW_OFFLOAD] tags in the conntrack table
  listing.
- Replace 'flow offload' by 'flow add' in example rulesets (preferred
  syntax).
- Describe existing cache limitations.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>