bpo-25381: Update explanation of exceptions in C. (GH-26838) * bpo-25381: Update explanation of exception globals This paragraph in extending/extending.rst was unchanged (except for NULL markup) since committed in 2007 Aug 15 for 2.6 alpha. * Respond to reviews and remove duplication. * Update Doc/extending/extending.rst Co-authored-by: Serhiy Storchaka <storchaka@gmail.com> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>