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: 134ef238

 ("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>