qpa: Document a requirement of the backing store implementation

Docuent the requirement that the alpha channels need to be properly
initialized by the implementation.

Change-Id: I03db81b44b43ea75feb1b983fb0725c65a3bd9f4
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/painting/qplatformbackingstore_qpa.cpp

@@ -114,6 +114,9 @@ QWindow* QPlatformBackingStore::window() const
     This function is called before painting onto the surface begins,
     with the \a region in which the painting will occur.
 
+    \note A platform providing a backing store with an alpha channel
+    needs to properly initialize the region to be painted.
+
     \sa endPaint(), paintDevice()
 */