add python api example and update doc

docs/en_US/Tutorial/HowToLaunchFromPython.rst

@@ -61,7 +61,7 @@ Now, you have successfully launched an NNI experiment. And you can type ``localh
 
 Example
 -------
-Below is an example for this new launching approach. You can also find this code in :githublink:`mnist-tfv2/launch.py <examples/trials/mnist-tfv2/launch.py>` .
+Below is an example for this new launching approach. You can also find this code in :githublink:`mnist-tfv2/launch.py <examples/trials/mnist-tfv2/launch.py>`.
 
 .. code-block:: python
 
@@ -90,6 +90,15 @@ Below is an example for this new launching approach. You can also find this code
 
     experiment.run(8081)
 
+View and Control Experiment with Python API
+-------------------------------------------
+We migrate the API in `nnicli` to this new launching approach.
+Launch the experiment by `Experiment.start()` instead of `Experiment.run()`, then you can use these APIs in interactive mode.
+`Experiment.run()` polls the experiment status and will automatically call `Experiment.stop()` when the experiment finished.
+`Experiment.start()` just launch a new experiment, so you need to manually stop the experiment by calling `Experiment.stop()`.
+If you launch the experiment by `nnictl` and also want to use these APIs, you can use `Experiment.connect()` to connect to an exsit experiment.
+An example with Jupyter Notebook has been provided, view :githublink:`python_api_quickstart.ipynb <examples/trials/sklearn/classification/python_api_quickstart.ipynb>`.
+
 API
 ---