Doc: Hide QPA classes

Mark QPlatformGraphicsBuffer and QSystemTrayIcon as internal.

Change-Id: I39e5b4f635fca21ab0544cd840a113514b6b1e99
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/kernel/qplatformgraphicsbuffer.cpp

@@ -63,6 +63,8 @@ QT_BEGIN_NAMESPACE
     QPlatformGraphicsBuffer is intended to be created by using platform specific
     APIs available from QtPlatformHeaders, or there might be accessor functions
     similar to the accessor function that QPlatformBackingstore has.
+
+    \internal
 */
 
 /*!

src/gui/kernel/qplatformsystemtrayicon.cpp

@@ -52,6 +52,7 @@ QT_BEGIN_NAMESPACE
     \inmodule QtGui
     \brief The QPlatformSystemTrayIcon class abstracts the system tray icon and interaction.
 
+    \internal
     \sa QSystemTrayIcon
 */
 
@@ -82,16 +83,10 @@ QT_BEGIN_NAMESPACE
      \sa activated()
 */
 
-/*!
-    \internal
- */
 QPlatformSystemTrayIcon::QPlatformSystemTrayIcon()
 {
 }
 
-/*!
-    \internal
- */
 QPlatformSystemTrayIcon::~QPlatformSystemTrayIcon()
 {
 }