Added HAL documentation note for out-of-bound hack in optical flow LK.

[asmorkalov]

• OpenCV and OpenCV/Carotene implementations are derived from the same root and expect +- winsize border around the actual image and its derivatives. The functions get cv::Mat with the padded buffer like ROI.
• OpenCV and OpenCV/Carotene Scharr derivatives have the same assumption in design and use +1 pixel border of the input image.
• The border is already taken into account during pyramid allocation. It's filled by the image pixels, if available or with copyMakeBorder. CopyMakeBorder call has ticky logic and allows passing ROI of the original image as destination.  The function recognizes it and fills the border, but does not make a copy. So the current implementation is efficient in this term.

Relates to:

• Added HAL interface for Lukas-Kanade optical flow #26143
• HAL interface for Sharr derivatives needed for Lukas-Kanade algorithm #26163

Pull Request Readiness Checklist

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

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• 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