The documentation that is required for the homework is the comments. The professor wants each block in the program to has it's own comment that describe what the code does.

//package for the code to be placed
package leo;
// required imports to run the application
import javafx.application.Application;
import javafx.collections.ObservableList;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.input.MouseButton;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
//class designed to extend the Application class
public class Exercise_15_15 extends Application {
//create an empty pane
Pane pane = new Pane();
@Override
public void start(Stage primaryStage) {
double width = 400;
double height = 400;
...