clearer to understand the boundary of internal buffers

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 OpenCV (BSD) License.
• [+] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• [+] The PR is proposed to proper branch
• [+] There is reference to 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