make it clear what services Applet provides in the apidox

svn path=/trunk/KDE/kdebase/workspace/libs/plasma/; revision=693718

applet.h

@@ -36,9 +36,20 @@ class DataEngine;
 class Package;
 
 /**
- * @short The base Applet (Plasmoid) class
+ * @short The base Applet class
  *
+ * Applet provides several important roles for add-ons widgets in Plasma.
  *
+ * First, it is the base class for the plugin system and therefore is the
+ * interface to applets for host applications. It also handles the life time
+ * management of data engines (e.g. all data engines accessed via
+ * Applet::datEngine(const QString&) are properly deref'd on Applet
+ * destruction), background painting (allowing for consistent and complex
+ * look and feel in just one line of code for applets), loading and starting
+ * of scripting support for each applet, providing access to the associated
+ * plasmoid package (if any) and access to configuration data.
+ *
+ * See techbase.kde.org for tutorial on writing Applets using this class.
  */
 class PLASMA_EXPORT Applet : public QObject, public Widget
 {