Documentation: Clarify trap's description

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: f3047ca01f12 ("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>