Add hint regarding alpha fbo's in the textureinthread example.

Change-Id: I92e6a7c2ca9e9f229b1fc463feea9f657ae18db9
Task-number: QTBUG-42486
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

examples/quick/scenegraph/textureinthread/threadrenderer.cpp

@@ -189,6 +189,8 @@ public slots:
         m_mutex.unlock();
         if (newId) {
             delete m_texture;
+            // note: include QQuickWindow::TextureHasAlphaChannel if the rendered content
+            // has alpha.
             m_texture = m_window->createTextureFromId(newId, size);
             setTexture(m_texture);