add border replicate note

modules/imgproc/include/opencv2/imgproc.hpp

@@ -1321,6 +1321,8 @@ The function smoothes an image using the median filter with the \f$\texttt{ksize
 \texttt{ksize}\f$ aperture. Each channel of a multi-channel image is processed independently.
 In-place operation is supported.
 
+@note The median filter uses BORDER_REPLICATE internally to cope with border pixels, see cv::BorderTypes
+
 @param src input 1-, 3-, or 4-channel image; when ksize is 3 or 5, the image depth should be
 CV_8U, CV_16U, or CV_32F, for larger aperture sizes, it can only be CV_8U.
 @param dst destination array of the same size and type as src.