doc: CLarify random sampling instructions

Extended the instructions to make it clear that the user must always explicitly include the background class.

weight_sampling_tutorial.ipynb

@@ -297,7 +297,9 @@
     "n_classes_source = 81\n",
     "# TODO: Set the indices of the classes that you want to pick for the sub-sampled weight tensors.\n",
     "#       In case you would like to just randomly sample a certain number of classes, you can just set\n",
-    "#       `classes_of_interest` to an integer instead of the list below.\n",
+    "#       `classes_of_interest` to an integer instead of the list below. Either way, don't forget to\n",
+    "#       include the background class. That is, if you set an integer, and you want `n` positive classes,\n",
+    "#       then you must set `classes_of_interest = n + 1`.\n",
     "classes_of_interest = [0, 3, 8, 1, 2, 10, 4, 6, 12]\n",
     "# classes_of_interest = 9 # Uncomment this in case you want to just randomly sub-sample the last axis instead of providing a list of indices.\n",
     "\n",