docs: net: dsa: document port_fast_age

The provided information about FDB flushing is not really up to date.
The DSA core automatically calls port_fast_age() when necessary, and
drivers should just implement that rather than hooking it to
port_bridge_leave, port_stp_state_set and others.

Fixes: 732f794c ("net: dsa: add port fast ageing")
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>