Documentation: networking: switchdev: clarify device driver behavior

This patch provides details on the expected behavior of switchdev
enabled network devices when operating in a "stand alone" mode, as well
as when being bridge members. This clarifies a number of things that
recently came up during a bug fixing session on the b53 DSA switch
driver.

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