Removes the comment about whether the _system function is needed - this is very useful.
I wasn't sure about the function _update_solution being called 'after each Newton iteration'. The iteration is the loop - more precisely, the function is called after 'each inner solve for the Newton increment'.
_systemfunction is needed - this is very useful._update_solutionbeing called 'after each Newton iteration'. The iteration is the loop - more precisely, the function is called after 'each inner solve for the Newton increment'.