Document Artifacts Expire date

Currently we have .gitlab-ci.yml documentation:

• http://docs.gitlab.com/ce/ci/yaml/README.html#artifacts-expire_in

It's good enough for the start, but we should also document user side of the feature.

Edit user docs in https://docs.gitlab.com/ce/user/project/pipelines/job_artifacts.html:

• You specify expire_in in your .gitlab-ci.yml,
• You can Keep your artifacts from expiring,
• The artifacts are marked as to be deleted after expire date,

Edit admin docs in https://docs.gitlab.com/ce/administration/job_artifacts.html:

• The artifacts are cleaned up by expire_build_artifacts_worker_cron which is configured by default to 50 * * * * (every hour at 50 minutes)
• That expire_build_artifacts_worker is a configuration of gitlab.yml (for source installations) and gitlab.rb (for Omnibus installations)