Fix and complete JavaDoc comments

src/main/java/it/polimi/ingsw/gc14/HelloController.java

@@ -3,10 +3,16 @@ package it.polimi.ingsw.gc14;
 import javafx.fxml.FXML;
 import javafx.scene.control.Label;
 
+/**
+ * Controller for the initial JavaFX view.
+ */
 public class HelloController {
     @FXML
     private Label welcomeText;
 
+    /**
+     * Handles the click on the hello button.
+     */
     @FXML
     protected void onHelloButtonClick() {
         welcomeText.setText("Welcome to JavaFX Application!");