added docs for MOG_GPU and MOG2_GPU
added docs for MOG_GPU and MOG2_GPU

opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,apavlenko/opencv