Documentation: Clarify trap's description

Alex noted that the below description might not be obvious to all users.
Clarify it by adding an example.

Fixes: f3047ca0 ("Documentation: Add devlink-trap documentation")
Reported-by: Alex Kushnarov <alexanderk@mellanox.com>
Reviewed-by: Alex Kushnarov <alexanderk@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/devlink-trap.rst

@@ -143,7 +143,8 @@ be added to the following table:
    * - ``port_list_is_empty``
      - ``drop``
      - Traps packets that the device decided to drop in case they need to be
-       flooded and the flood list is empty
+       flooded (e.g., unknown unicast, unregistered multicast) and there are
+       no ports the packets should be flooded to
    * - ``port_loopback_filter``
      - ``drop``
      - Traps packets that the device decided to drop in case after layer 2