docs: add demo covariance matrix

docs/usage/step3.ipynb

@@ -469,6 +469,37 @@
     "As can be seen, the values of the optimized parameters in the result are again comparable to the original values that are contained in the model ({attr}`.SympyModel.parameters`):"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "#### Covariance matrix"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Each of the {mod}`.optimizer`s offer more specific information about the fit result. This information can be accessed with {attr}`.FitResult.specifics`. A common example would be to get the `~iminuit.Minuit.covariance` matrix:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "covariance_matrix = fit_result.specifics.covariance\n",
+    "covariance_matrix"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "#### AIC and BIC"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

src/tensorwaves/interface.py

@@ -156,6 +156,9 @@ class FitResult:  # pylint: disable=too-many-instance-attributes
 
     - `iminuit.Minuit`
     - `scipy.optimize.OptimizeResult`
+
+    This way, you can for instance get the `~iminuit.Minuit.covariance` matrix.
+    See also :ref:`/usage/step3:Covariance matrix`.
     """
 
     @parameter_errors.validator  # pyright: reportOptionalMemberAccess=false