> I don't think we should document this level of detail

But a lot of it -- most of it -- is also strange behavior that no one would expect without reading about it somewhere. Most users of subprocess.Popen() will never wade through the documentation of CreateProcessW, SearchPathW, ShellExecuteExW, and cmd.exe -- to the extent that the behavior is usefully and correctly documented by Microsoft.

One of the reasons I bother writing it out in detail here is to make the case for always using shutil.which(), regardless of the value of `shell` (unless shell=True is being used beyond the filesystem scope). We have complete control of the implementation and documentation of shutil.which(). I don't want that advice to be narrowed down to just talking about running "python".