Make it clearer that when using the floating button, the 'floating button action' is used instead of the global native / legacy method setting

As per discussions on https://github.com/cvzi/ScreenshotTile/issues/682 and https://github.com/cvzi/ScreenshotTile/issues/683, it is not clear that the native / legacy method setting on the main screen does not control the behaviour of the floating button