[3.12] gh-128017: Make a note that sys variables are read-only (GH-128887) (#128909)

gh-128017: Make a note that sys variables are read-only (GH-128887)
(cherry picked from commit 313b96eb8b8d0ad3bac58d633822a0a3705ce60b)

Co-authored-by: Srinivas Reddy Thatiparthy (తాటిపర్తి శ్రీనివాస్  రెడ్డి) <thatiparthysreenivas@gmail.com>