Documentation: net: dsa: Describe DSA switch configuration

Document DSA tagged and VLAN based switch configuration by showcases.

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>