docs: net: dsa: describe issues with checksum offload

DSA tags before IP header (categories 1 and 2) or after the payload (3)
might introduce offload checksum issues.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>