Error in the documentation for cv::getRectSubPix. #9788 (#9871)

* Error in the documentation for cv::getRectSubPix. #9788
The function name is corrected to GetRectSubPix since, it uses the notation
of src, dst and center. Also added the number of channel assertion criteria.

* Error in the documentation for cv::getRectSubPix. #9788
Replace dst with patch in the formula, reverted function name to
getRectSubPix, removed BorderTypes comment line due to no explicit call
to the function found.

* Error in the documentation for cv::getRectSubPix. #9788
Replace dst with patch in the formula, reverted function name to
getRectSubPix, removed BorderTypes comment line due to no explicit call
to the function found.

modules/imgproc/include/opencv2/imgproc.hpp

@@ -2470,13 +2470,12 @@ CV_EXPORTS_W Mat getAffineTransform( InputArray src, InputArray dst );
 
 The function getRectSubPix extracts pixels from src:
 
-\f[dst(x, y) = src(x +  \texttt{center.x} - ( \texttt{dst.cols} -1)*0.5, y +  \texttt{center.y} - ( \texttt{dst.rows} -1)*0.5)\f]
+\f[patch(x, y) = src(x +  \texttt{center.x} - ( \texttt{dst.cols} -1)*0.5, y +  \texttt{center.y} - ( \texttt{dst.rows} -1)*0.5)\f]
 
 where the values of the pixels at non-integer coordinates are retrieved using bilinear
-interpolation. Every channel of multi-channel images is processed independently. While the center of
-the rectangle must be inside the image, parts of the rectangle may be outside. In this case, the
-replication border mode (see cv::BorderTypes) is used to extrapolate the pixel values outside of
-the image.
+interpolation. Every channel of multi-channel images is processed independently. Also
+the image should be a single channel or three channel image. While the center of the
+rectangle must be inside the image, parts of the rectangle may be outside.
 
 @param image Source image.
 @param patchSize Size of the extracted patch.