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>