sfc: document TC-to-EF100-MAE action translation concepts

Includes an explanation of the lifetime of the 'cursor' action-set `act`.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>