[3.12] gh-101100: Clean up Doc/c-api/exceptions.rst and Doc/c-api/sys.rst (GH-114825) (GH-115308)

(cherry picked from commit e1552fd19de17e7a6daa3c2a6d1ca207bb8eaf8e)

Co-authored-by: Skip Montanaro <skip.montanaro@gmail.com>