Documentation: networking: dsa: Describe port_vlan_filtering

Described what the port_vlan_filtering function is supposed to
accomplish.

Fixes: fb2dabad

 ("net: dsa: support VLAN filtering switchdev attr")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>