Silly me just spent about an hour trying to get the download cache working as advised in the Further Configuration of Pip and Virtualenv section before finally learning on Stack Overflow that pip caches by default since version 6.0 or so.

I don't know if that whole subsection about the download cache is therefore obsolete, but I think there should definitely at least be a sentence indicating the change of behavior in newer pip versions to save other readers from the same misadventure.