Merge pull request #27985 from happy-capybara-man:docs/fix-mat-clone-js

docs(js): Fix Mat.clone() documentation to use mat_clone() for deep copy #27985

- Update code example to use ```mat_clone()``` instead of ```clone()```
- Add explanatory note about shallow copy issue due to Emscripten embind
Problem
- OpenCV.js documentation shows ```Mat.clone()``` usage, but this method performs shallow copy instead of deep copy due to Emscripten embind limitations, causing unexpected behavior where modifications to cloned matrices affect the original.

Related Issues and PRs
- Fixes documentation aspect of issue #27572
- Related to PR #26643 (js_clone_fix)

### 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

samples/dnn/js_face_recognition.html

@@ -132,7 +132,7 @@ function main() {
       var cell = document.getElementById("targetNames").insertCell(0);
       cell.innerHTML = name;
 
-      persons[name] = face2vec(face).clone();
+      persons[name] = face2vec(face).mat_clone();
 
       var canvas = document.createElement("canvas");
       canvas.setAttribute("width", 112);