docs: QApplication::startDragDistance default value is 10

This comes from QPlatformTheme::defaultThemeHint().

Change-Id: I12a9add3af65e819a06b66d958acb8f21cfe0e13
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/widgets/kernel/qapplication.cpp

@@ -2691,7 +2691,8 @@ void QApplication::setStartDragDistance(int l)
 
     Qt uses this value internally, e.g. in QFileDialog.
 
-    The default value is 4 pixels.
+    The default value (if the platform doesn't provide a different default)
+    is 10 pixels.
 
     \sa startDragTime(), QPoint::manhattanLength(), {Drag and Drop}
 */