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: 732f794c1b ("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>

Documentation/networking/dsa/dsa.rst

@@ -738,15 +738,11 @@ Bridge layer
 - ``port_bridge_leave``: bridge layer function invoked when a given switch port is
   removed from a bridge, this function should do what's necessary at the
   switch level to deny the leaving port from ingress/egress traffic from the
-  remaining bridge members. When the port leaves the bridge, it should be aged
-  out at the switch hardware for the switch to (re) learn MAC addresses behind
-  this port.
+  remaining bridge members.
 
 - ``port_stp_state_set``: bridge layer function invoked when a given switch port STP
   state is computed by the bridge layer and should be propagated to switch
-  hardware to forward/block/learn traffic. The switch driver is responsible for
-  computing a STP state change based on current and asked parameters and perform
-  the relevant ageing based on the intersection results
+  hardware to forward/block/learn traffic.
 
 - ``port_bridge_flags``: bridge layer function invoked when a port must
   configure its settings for e.g. flooding of unknown traffic or source address
@@ -775,6 +771,12 @@ Bridge layer
 - ``port_bridge_tx_fwd_unoffload``: bridge layer function invoked when a driver
   leaves a bridge port which had the TX forwarding offload feature enabled.
 
+- ``port_fast_age``: bridge layer function invoked when flushing the
+  dynamically learned FDB entries on the port is necessary. This is called when
+  transitioning from an STP state where learning should take place to an STP
+  state where it shouldn't, or when leaving a bridge, or when address learning
+  is turned off via ``port_bridge_flags``.
+
 Bridge VLAN filtering
 ---------------------