[3.14] gh-148320: document that `import sys.monitoring` raises `ModuleNotFoundError` (GH-148365) (#148385)

gh-148320: document that `import sys.monitoring` raises `ModuleNotFoundError` (GH-148365)
(cherry picked from commit d7c9f1877cbd733d6c0fda395a9fcf78171f51b0)

Co-authored-by: Jonathan Dung <jonathandung@yahoo.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>