[3.14] gh-141004: Document `Py_BUILD_ASSERT*` macros (GH-141266) (GH-141269)

gh-141004: Document `Py_BUILD_ASSERT*` macros (GH-141266)
(cherry picked from commit 0ac890bea79d3e0162c8909b0999f626f1141d89)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>