Follow-up from "docs: cache:paths - make clear, that only paths within the project folder can be cached."

The following discussion from !18028 (merged) should be addressed:

• @steveazz started a discussion: (+1 comment)

  That looks good to me 🚀 Thank you @strowi for adding it 🙇

  @marcel.amirault we might want to do a follow-up to add this kind of information in https://docs.gitlab.com/ee/ci/caching/ if you think it makes sense to have that kind of information duplicated.