Release notes 12-9

@mhoeber @lamagnifica @catong Please review.

@clintonb Since you're release lord (should that be capitalized?) this week, you get a sneak peek at the release notes. You don't have to do a thorough review, but feel free to read through them and let me know if anything seems amiss.
no. 
The strategy on my end was to describe what necessitated this change once, when the change was made, and put it in the release notes. In the doc, I describe what *is* (essentially, the 1st and last sentences as above).