I could imagine that several very long Mathoverflow answers have eventually turned into expository papers. For example, I have posted this (unpublished) expository paper on the arxiv about various notions of zero-dimensionality as a response to a MO question. What are some examples of some answers to Mathoverflow questions which have eventually grown into expository papers or other forms of mathematical exposition (such as a section in a textbook or even a piece of software)?

In this question, people gave examples of good original research that has arisen from mathoverflow answers, but I want to now see examples of published mathematical exposition which have arisen as the result of Mathoverflow answers.