Fix an inaccurate comment.

Fix an inaccurate comment.

latin_solver_diff_set takes a boolean input parameter to indicate
whether the 'Extreme'-level variant of set elimination is permitted.
But it's commented in the header file as having a boolean _output_
parameter which it sets if it _ended up_ using that variant. Probably
the latter was how it worked in an early draft, and I changed my mind
later without changing the comment.