_preprocess_data interferes with the docstrings Notes section. It appends a Notes section (1) in the wrong section; (2) in the rendering, it overwrites the notes section of the function.

In general, I think matplotlib is trying to be way too smart in appending/replacing stuff in the docstrings. It makes contributing to the documentation (and matplotlib in general) quite hard and often interferes with sphinx or other tools we use. We should try to be more standard.