steady: allow for non_zero option

Closes #1945 (closed), related to preprocessor!109 (merged)

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

@@ -2915,6 +2915,9 @@ Finding the steady state with Dynare nonlinear solver
        along. Iteration will cease when the attempted step size is smaller than
        ``tolx``. Default: ``eps^(2/3)``
+    .. option:: non_zero
+
+       See :opt:`non_zero`.
     .. _solvalg: