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 <[email protected]> Co-authored-by: Peter Bierma <[email protected]>