Add explanation about set and alter in ex2

examples/ex2.ipynb

@@ -347,7 +347,15 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The load values can be manipulated in the model `PQ`."
+    "The load values can be manipulated in the model `PQ`.\n",
+    "\n",
+    "Note the difference between `Model.set` and `Model.alter`:\n",
+    "\n",
+    "`set`: This method ***WILL NOT*** modify the input values from the case file that have not been converted to the system base. As a result, changes applied by this method ***WILL NOT*** affect the dumped case file.\n",
+    "\n",
+    "`alter`: If the method operates on an input parameter, the new data ***should be in the same base as that in the input file***. This function will convert ``value`` to per unit in the system base whenever necessary. The values for storing the input data, i.e., the parameter's ``vin`` field, will be overwritten. As a result, altered values ***WILL BE*** reflected in the dumped case file.\n",
+    "\n",
+    "Besides, `Group` also has method `set` but has no `alter`."
    ]
   },
   {