docs: net: dsa: mention that VLANs are now refcounted on shared ports

The blamed commit updated the way in which VLANs are handled at the
cross-chip notifier layer and didn't update the documentation to say
that. Fix it.

Fixes: 134ef23 ("net: dsa: add explicit support for host bridge VLANs")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/dsa/dsa.rst

@@ -951,7 +951,13 @@ Bridge VLAN filtering
   allowed.
 
 - ``port_vlan_add``: bridge layer function invoked when a VLAN is configured
-  (tagged or untagged) for the given switch port..
+  (tagged or untagged) for the given switch port. The CPU port becomes a member
+  of a VLAN only if a foreign bridge port is also a member of it (and
+  forwarding needs to take place in software), or the VLAN is installed to the
+  VLAN group of the bridge device itself, for termination purposes
+  (``bridge vlan add dev br0 vid 100 self``). VLANs on shared ports are
+  reference counted and removed when there is no user left. Drivers do not need
+  to manually install a VLAN on the CPU port.
 
 - ``port_vlan_del``: bridge layer function invoked when a VLAN is removed from the
   given switch port