Manual: give more details for stack_solve_algo=0

Explain that the solver is a direct sparse LU, to differentiate it from
stack_solve_algo={2,3,4} which use the same Newton algorithm but with different
solvers.

doc/manual/source/the-model-file.rst

@@ -3625,8 +3625,9 @@ speed-up on large models.
 
            ``0``
 
-               Newton method to solve simultaneously all the equations for
-               every period, using sparse matrices (Default).
+               Use a Newton algorithm with a direct sparse LU solver at each
+               iteration, applied on the stacked system of all the equations at
+               every period (Default).
 
            ``1``