I know those sorts of fictional facts as False Documents included in novels. A false document is a contrived document that is real in a fictional world. Douglas Adams' The Hitchhiker's Guide to the Galaxy references a false document by the same name, for example. Sometimes false documents take on a real-world life on their own. The textbook Fantastic Beasts and Where to Find Them introduced in the first Harry Potter installment was published in the real world 2001.
False documents occur throughout the opus of literature as letters, diaries, novels, textbooks, state papers, newspaper, magazine, encyclopedia articles, fictional person prefaces, introductions, prologues, etc. False documents aren't limited to book and digest publications. They can be any invented document. ID card, cereal box, book cover blurb, manual, TV commercial, speech, platitude, quotation, billboard, schematic, and so on.
One thing I've not seen that I can think of is a false document known to be a false document arising in a fictional world and reported as a factual document of an internal fictional world. No, wait, I have, sort of, Philip K. Dick's The Man in the High Castle references a novel, The Grasshopper Lies Heavy, which references a fictional history that is the real-world factual history.
Spread the love of written word.