doc: suggest setting HOME=/dev/null

Otherwise Vim will use it in &runtimepath, which loads plugins from
~/.vim/plugin.

Instead of /dev/null $PWD or something else could be used, but then
`set viminfo=` should get suggested, too.

While at it, I think the docs should suggest using a `vimrc` file, instead of the `<(cat ..)` blob.
Plugin authors will use it like this, and it makes it more difficult to maintain, compared to using a `test/vimrc` file.