Merge pull request #27730 from Kumataro:fix27729

doc: fix doxygen warnings for imgcodecs, flann and objdetect #27730

Close https://github.com/opencv/opencv/issues/27729

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/objdetect/doc/objdetect.bib

@@ -16,7 +16,8 @@
   title = {{AprilTag} 2: Efficient and robust fiducial detection},
   booktitle = {Proceedings of the {IEEE/RSJ} International Conference on Intelligent Robots and Systems {(IROS)}},
   year = {2016},
-  month = {October}
+  month = {October},
+  url = {https://april.eecs.umich.edu/pdfs/wang2016iros.pdf}
 }
 
 @mastersthesis{Xiangmin2015research,