Documentation: networking: dsa: add paragraph for the HSR/PRP offload

Add a short summary of the methods that a driver writer must implement
for offloading a HSR/PRP network interface.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: George McCollister <george.mccollister@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>