Documentation: networking: dsa: add paragraph for the LAG offload

Documentation: networking: dsa: add paragraph for the LAG offload

Add a short summary of the methods that a driver writer must implement
for offloading a link aggregation group, and what is still missing.

Cc: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>