[3.13] gh-129407: Clarify that a `SystemError` isn't always CPython's fault (GH-129410) (#129610)

gh-129407: Clarify that a `SystemError` isn't always CPython's fault (GH-129410)
(cherry picked from commit 39b754a35976924f6df46cd475e889bcf8598ca1)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>