Add documentation for cross project artifacts

(cherry picked from commit fbb65a15910122689f307fe19dff042ef648a32d)

changelogs/unreleased/enable-cross-project-artifacts.yml

+---
+title: Download cross-project artifacts by using needs keyword in the CI file
+merge_request: 22161
+author:
+type: added

doc/ci/yaml/README.md

@@ -2385,6 +2385,51 @@ rspec:
     - build_job_3
 ```
 
+#### Cross project artifact downloads with `needs` **(PREMIUM)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/14311) in GitLab v12.7.
+
+`needs` can be used to download artifacts from up to five jobs in pipelines on
+[other refs in the same project](#artifact-downloads-between-pipelines-in-the-same-project),
+or pipelines in different projects:
+
+```yaml
+build_job:
+  stage: build
+  script:
+    - ls -lhR
+  needs:
+    - project: group/project-name
+      job: build-1
+      ref: master
+      artifacts: true
+```
+
+`build_job` will download the artifacts from the latest successful `build-1` job
+on the `master` branch in the `group/project-name` project.
+
+##### Artifact downloads between pipelines in the same project
+
+`needs` can be used to download artifacts from different pipelines in the current project
+by setting the `project` keyword as the current project's name, and specifying a ref.
+In the example below, `build_job` will download the artifacts for the latest successful
+`build-1` job with the `other-ref` ref:
+
+```yaml
+build_job:
+  stage: build
+  script:
+    - ls -lhR
+  needs:
+    - project: group/same-project-name
+      job: build-1
+      ref: other-ref
+      artifacts: true
+```
+
+NOTE: **Note:**
+Downloading artifacts from jobs that are run in [`parallel:`](#parallel) is not supported.
+
 ### `coverage`
 
 > [Introduced][ce-7447] in GitLab 8.17.

ee/app/models/ee/ci/build.rb

@@ -120,7 +120,7 @@ module EE
       override :cross_dependencies
       def cross_dependencies
         return [] unless user_id
-        return [] unless ::Feature.enabled?(:cross_project_need_artifacts, project, default_enabled: false)
+        return [] unless ::Feature.enabled?(:cross_project_need_artifacts, project, default_enabled: true)
         return [] unless project.feature_available?(:cross_project_pipelines)
 
         cross_dependencies_relationship