Add mention about model_set_axis=False in Model's docstring - #7143

________________________________ test_2d_model _________________________________
[gw0] linux -- Python 3.6.4 /opt/python/cp36-cp36m/bin/python

    @pytest.mark.skipif('not HAS_SCIPY')
    def test_2d_model():
        # 2D model with LevMarLSQFitter
        gauss2d = models.Gaussian2D(10.2, 4.3, 5,2, 1.2, 1.4)
        fitter = fitting.LevMarLSQFitter()
        X = np.linspace(-1, 7)
        Y = np.linspace(-1, 7)
        x, y = np.meshgrid(X, Y)
        z = gauss2d(x, y)
        w = np.ones(x.size)
        w.shape = x.shape
        rsn = RandomState(1234567890)
        n = rsn.randn(x.size)
        n = np.random.randn(x.size)
        n.shape = x.shape
        m = fitter(gauss2d, x, y, z+2*n, weights=w)
        utils.assert_allclose(m.parameters, gauss2d.parameters, rtol=1e-1)
        m = fitter(gauss2d, x, y, z+2*n, weights=None)
        utils.assert_allclose(m.parameters, gauss2d.parameters, rtol=1e-1)
        # 2D model with LevMarLSQFitter, fixed constraint
        gauss2d.x_stddev.fixed = True
        m = fitter(gauss2d, x, y, z+2*n, weights=w)
        utils.assert_allclose(m.parameters, gauss2d.parameters, rtol=1e-1)
        m = fitter(gauss2d, x, y, z+2*n, weights=None)
        utils.assert_allclose(m.parameters, gauss2d.parameters, rtol=1e-1)
        # Polynomial2D, col_fit_deriv=False
        p2 = models.Polynomial2D(1, c0_0=1, c1_0=1.2, c0_1=3.2)
        z = p2(x, y)
        m = fitter(p2, x, y, z + 2 * n, weights=None)
>       utils.assert_allclose(m.parameters, p2.parameters, rtol=1e-1)
E       AssertionError: 
E       Not equal to tolerance rtol=0.1, atol=0
E       
E       (mismatch 33.33333333333333%)
E        x: array([1.113545, 1.194334, 3.162268])
E        y: array([1. , 1.2, 3.2])

astropy/modeling/tests/test_constraints.py:511: AssertionError