docs: net: dsa: re-explain what port_fdb_dump actually does

Switchdev has changed radically from its initial implementation, and the
currently provided definition is incorrect and very confusing.

Rewrite it in light of what it actually does.

Fixes: 2bedde1a ("net: dsa: Move FDB dump implementation inside DSA")
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>