gh-141004: Document `PyException_HEAD` by StanFromIreland · Pull Request #141522 · python/cpython

I would assume that's being used there because they saw how we define exceptions internally and must have thought, "Oh, so that's how you do that". I'd be in favor of hard-deprecating this one, since it will likely create an actual maintenance burden if we want to change exceptions to use PEP 697 someday.