Add docs for change of Cache/Artifact restore order

Add to https://docs.gitlab.com/ce/ci/yaml/README.html#artifacts

Prior to 9.2 Artifacts were restored before Caches. Which could results in an edge case where artifact files could be overridden by old cache files if the same cache and artifact path was used.

Changes in 9.2 Artifacts are restored after the cache, ensuring the the latest files from the artifact are used and not overridden by the cache in this edge case.