gh-98250: Improve documentation about change to Enum.__format__

[mdboom]

• Issue: Enum.__str__ change could be improved in What's New #98250

[JelleZijlstra]

Might be worth adding explicitly that this affects enums included in f-strings, since that may not be obvious to a lot of readers.

[CAM-Gerlach]

Hey, I missed seeing this before I submitted #98295 , but it turns out I noticed a similar issue, as the What's New was not entirely clear on that point, and had already made some enhancements in that regard. I went ahead and made some further improvements and expansions there to more comprehensively address #98250 and incorporate the intent of the changes here, alongside @JelleZijlstra 's suggestion and a number of additional fixes and clarifications.

As that PR essentially rewrites that section and will thus conflict with this one, which I'm still not sure is quite right (as ReprEnum appears to do something quite different from the old __format__ behavior, and there are also still a few smaller points of confusion here), my suggestion would be to consolidate on that one (especially since we don't have much time left before the release), and I'd welcome your review and suggestions over there.

[mdboom]

Closing in favor of the more comprehensive #98295.