Document restrictions on cache and artifact paths

What does this MR do?

Includes a note on valid paths for cache and artifacts in the place users are most likely to learn about the feature.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

We get regular support requests relating to cache and artifacts not being picked up outside the project root. It's intended behaviour, and should be documented.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1792

(plus others)