Added Javadoc for handleOpenFilesAction(...) methods.

app/src/main/java/com/oracle/javafx/scenebuilder/app/SceneBuilderApp.java

@@ -497,11 +497,24 @@ private void createEmptyDocumentWindow() {
         newWindow.updateWithDefaultContent();
     }
 
+    /**
+     * By default all necessary actions to open a single file or a group of files take place in this method.
+     * If it is required to perform certain actions after successfully loading all files, please use {@code handleOpenFilesAction(List<String> files, Runnable onSuccess)} instead.
+     * 
+     * All error handling takes place here within, there is no way yet to access exceptional results and to work with them.
+     */
     @Override
     public void handleOpenFilesAction(List<String> files) {
         handleOpenFilesAction(files, null);
     }
 
+    /**
+     * As file loading errors are handled within this method (all exceptions are handled within), it can be helpful to be able to run a certain action after successful file loading (e.g. closing a certain stage).
+     * For this case this method offers the argument {@code Runnable onSuccess} which will be executed after successful file open activity. The {@code Runnable onSuccess} is only ran once, despite how many files have been loaded.
+     *  
+     * @param files List of Strings denoting file paths to be opened
+     * @param onSuccess {@link Runnable} to be executed after all files have been opened successfully
+     */
     public void handleOpenFilesAction(List<String> files, Runnable onSuccess) {
         assert files != null;
         assert files.isEmpty() == false;