Documentation: networking: dsa: document the port_bridge_flags method

The documentation was already lagging behind by not mentioning the old
version of port_bridge_flags (port_set_egress_floods). So now we are
skipping one step and just explaining how a DSA driver should configure
address learning and flooding settings.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>