Documentation: networking: dsa: Describe port_vlan_filtering

Described what the port_vlan_filtering function is supposed to
accomplish.

Fixes: fb2dabad69 ("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>

Documentation/networking/dsa/dsa.txt

@@ -535,6 +535,16 @@ Bridge layer
 Bridge VLAN filtering
 ---------------------
 
+- port_vlan_filtering: bridge layer function invoked when the bridge gets
+  configured for turning on or off VLAN filtering. If nothing specific needs to
+  be done at the hardware level, this callback does not need to be implemented.
+  When VLAN filtering is turned on, the hardware must be programmed with
+  rejecting 802.1Q frames which have VLAN IDs outside of the programmed allowed
+  VLAN ID map/rules.  If there is no PVID programmed into the switch port,
+  untagged frames must be rejected as well. When turned off the switch must
+  accept any 802.1Q frames irrespective of their VLAN ID, and untagged frames are
+  allowed.
+
 - port_vlan_prepare: bridge layer function invoked when the bridge prepares the
   configuration of a VLAN on the given port. If the operation is not supported
   by the hardware, this function should return -EOPNOTSUPP to inform the bridge