FAQ XDG section needs updating

[marckhouzam]

I was browsing the FAQ and noticed the XDG section (https://helm.sh/docs/faq/#xdg-base-directory-support) mentions that HELM_HOME is set which I don't believe is the case.

It also mentions HELM_PATH_CACHE and other such variables which I also don't believe exist.

I'm sorry that I don't have the chance to post an actual PR.