docs: net: dsa: document port_fast_age

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: 732f794c1baf ("net: dsa: add port fast ageing")
Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>