videoio: add note about image BGR format VideoWriter::write()

modules/videoio/include/opencv2/videoio.hpp

@@ -905,7 +905,7 @@ public:
 
     /** @brief Writes the next video frame
 
-    @param image The written frame
+    @param image The written frame. In general, color images are expected in BGR format.
 
     The function/method writes the specified image to video file. It must have the same size as has
     been specified when opening the video writer.