Clarify needs behavior with artifacts

doc/ci/yaml/README.md

@@ -1983,6 +1983,10 @@ To disable directed acyclic graphs (DAG), set the limit to `0`.
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14311) in GitLab v12.6.
 
+When a job uses `needs`, it no longer downloads all artifacts from previous stages
+by default, because jobs with `needs` can start before earlier stages complete. With
+`needs` you can only download artifacts from the jobs listed in the `needs:` configuration.
+
 Use `artifacts: true` (default) or `artifacts: false` to control when artifacts are
 downloaded in jobs that use `needs`.
 
@@ -3072,6 +3076,13 @@ The artifacts are sent to GitLab after the job finishes. They are
 available for download in the GitLab UI if the size is not
 larger than the [maximum artifact size](../../user/gitlab_com/index.md#gitlab-cicd).
 
+By default, jobs in later stages automatically download all the artifacts created
+by jobs in earlier stages. You can control artifact download behavior in jobs with
+[`dependencies`](#dependencies).
+
+When using the [`needs`](#artifact-downloads-with-needs) keyword, jobs can only download
+artifacts from the jobs defined in the `needs` configuration.
+
 Job artifacts are only collected for successful jobs by default, and
 artifacts are restored after [caches](#cache).