ovis: document the supported environment variables

 * @param title window title

 * @param size size of the window

 * @param flags a combination of @ref SceneSettings

 *

 * Furthermore, the behavior is controlled by the following environment variables

 * - OPENCV_OVIS_VERBOSE_LOG: print all of OGRE log output

 * - OPENCV_OVIS_RENDERSYSTEM: the name of the OGRE RenderSystem to use

 * - OPENCV_OVIS_NOVSYNC: disable VSYNC for all windows

 */

CV_EXPORTS_W Ptr<WindowScene> createWindow(const String& title, const Size& size,

                                           int flags = SCENE_INTERACTIVE | SCENE_AA);