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)

doc/ci/yaml/README.md

@@ -159,7 +159,8 @@ Variables can be also defined on [job level](#job-variables).
 > Introduced in GitLab Runner v0.7.0.
 `cache` is used to specify a list of files and directories which should be
-cached between builds.
+cached between builds. You can only use paths that are within the project
+workspace.
 **By default the caching is enabled per-job and per-branch.**
@@ -606,8 +607,8 @@ You can see a simple example at https://gitlab.com/gitlab-examples/review-apps-n
 > - Build artifacts are only collected for successful builds by default.
 `artifacts` is used to specify a list of files and directories which should be
-attached to the build after success. To pass artifacts between different builds,
-see [dependencies](#dependencies).
+attached to the build after success. You can only use paths that are within the
+project workspace. To pass artifacts between different builds, see [dependencies](#dependencies).
 Below are some examples.