[SystemTray] Explain in comments why we need wrapping applet

Add brief explanation why we need thin wrapping applet.

applets/systemtray/container/package/contents/ui/main.qml

@@ -22,6 +22,7 @@ import QtQuick.Layouts 1.1
 import org.kde.plasma.core 2.0 as PlasmaCore
 import org.kde.plasma.plasmoid 2.0
 
+//SystemTray is a Containment. To have it presented as a widget in Plasma we need thin wrapping applet
 Item {
     id: root
 
@@ -32,6 +33,8 @@ Item {
     Plasmoid.preferredRepresentation: Plasmoid.fullRepresentation
     Plasmoid.status: internalSystray ? internalSystray.status : PlasmaCore.Types.UnknownStatus
 
+    //synchronize state between SystemTray and wrapping Applet
+    //the "expanded" property is not really used now, but let's keep it in sync for consistency
     Plasmoid.onExpandedChanged: {
         if (internalSystray && !plasmoid.expanded) {
             internalSystray.expanded = false;

applets/systemtray/container/systemtraycontainer.h

@@ -26,6 +26,11 @@
 
 class QQuickItem;
 
+/**
+ * @brief Thin wrapping 'Plasma::Applet' for SystemTray.
+ *
+ * SystemTray is of 'Plasma::Containment' type. To have it presented as a widget in Plasma we need a wrapping applet.
+ */
 class SystemTrayContainer : public Plasma::Applet
 {
     Q_OBJECT