Manual: fix math formula in “markowitz” option description

By the way, add some explanatory words.

[skip ci]

(cherry picked from commit 3e6b6c87)

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

@@ -2946,7 +2946,7 @@ Finding the steady state with Dynare nonlinear solver
     .. option:: markowitz = DOUBLE
-       Value of the Markowitz criterion (:math:(0,\infty)`) used to select the
+       Value of the Markowitz criterion (in the interval :math:`(0,\infty)`) used to select the
        pivot with sparse Gaussian elimination (``solve_algo = 5``). This criterion 
        governs the tradeoff between selecting the pivot resulting in the most 
        accurate solution (low ``markowitz`` values) and the one that preserves