Merge pull request opencv#18022 from SoheibKadi:Update_CornerSubPix_Documentation

Author: alalek

doc/opencv.bib

@@ -1238,3 +1238,10 @@ @Article{Wu2009
   number={2},
   pages={117-135},
 }
+@inproceedings{forstner1987fast,
+  title={A fast operator for detection and precise location of distincs points, corners and center of circular features},
+  author={FORSTNER, W},
+  booktitle={Proc. of the Intercommission Conference on Fast Processing of Photogrammetric Data, Interlaken, Switzerland, 1987},
+  pages={281--305},
+  year={1987}
+}

modules/imgproc/include/opencv2/imgproc.hpp

@@ -1861,8 +1861,8 @@ CV_EXPORTS_W void preCornerDetect( InputArray src, OutputArray dst, int ksize,
 
 /** @brief Refines the corner locations.
 
-The function iterates to find the sub-pixel accurate location of corners or radial saddle points, as
-shown on the figure below.
+The function iterates to find the sub-pixel accurate location of corners or radial saddle
+points as described in @cite forstner1987fast, and as shown on the figure below.
 
 ![image](pics/cornersubpix.png)