Document BIM CutPlane selection handling

[slendertaker]

Summary

• add a BIM release note for the CutPlane task-panel selection fix
• document that the original selection is kept for the final cut even if objects are deselected during preview
• update both root and English release-note pages

Verification

• git diff --check upstream/main..HEAD

Source: FreeCAD/FreeCAD#29699

AI-assisted with OpenAI Codex; I reviewed the final diff before submitting.

[Reqrefusion]

Do not add T tags yourself; only modify the files in the wiki root directory, do not touch the translation files. Please check the latest version and ensure that the information you are adding has not been added previously. Also, be sure to cite the source of your change. Regression corrections should not be added to the documentation. And it should never contain information that the end user shouldn't know.