Better doc for WARP_RELATIVE_MAP

[chacha21]

Pull Request Readiness Checklist

Relates to https://github.com/opencv/opencv/issues/24603

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

[chacha21]

My bad, the CUDA version is not yet committed and accepted ! https://github.com/opencv/opencv_contrib/pull/3600