Serhiy's point about how special this is is very valid. It's so special that I can't figure out where to document it. f-strings are really only documented in Doc/reference/lexical_analysis.rst, and !d details, especially the format/repr distinction, seems like too much information for that document. But I could be wrong about that.

And since this feature can't be used in str.format(), it can't be documented in Doc/library/string.rst. In fact, it should contain a note about !d not applying.

Rather than make the documentation worse, I think I'll just open a separate issue for it when I commit this. Maybe someone else will have some ideas.