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>

Documentation/networking/dsa/dsa.rst

@@ -970,9 +970,12 @@ Bridge VLAN filtering
   the specified MAC address from the specified VLAN ID if it was mapped into
   this port forwarding database
 
-- ``port_fdb_dump``: bridge layer function invoked with a switchdev callback
-  function that the driver has to call for each MAC address known to be behind
-  the given port. A switchdev object is used to carry the VID and FDB info.
+- ``port_fdb_dump``: bridge bypass function invoked by ``ndo_fdb_dump`` on the
+  physical DSA port interfaces. Since DSA does not attempt to keep in sync its
+  hardware FDB entries with the software bridge, this method is implemented as
+  a means to view the entries visible on user ports in the hardware database.
+  The entries reported by this function have the ``self`` flag in the output of
+  the ``bridge fdb show`` command.
 
 - ``port_mdb_add``: bridge layer function invoked when the bridge wants to install
   a multicast database entry. If the operation is not supported, this function