gh-141004: Reorganize and reword the 'Useful macros' section (GH-144471)

- Group the macros
- Roughly order them to put the most important ones first
- Add expansions where it makes sense; especially if there's
  an equivalent in modern C or a common compiler

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>