ovis: document the supported environment variables

Author: paroj

modules/ovis/include/opencv2/ovis.hpp

@@ -248,6 +248,11 @@ CV_EXPORTS_W void addResourceLocation(const String& path);
  * @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);