Merge branch 'clarify-cache-and-artifact-paths' into 'master'

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

## 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

See merge request !6892