Merge branch 'docs/improve-trigger-strategy-doc' into 'master'

Clarify the usage of `trigger:strategy`

See merge request gitlab-org/gitlab!23583

doc/ci/yaml/README.md

@@ -2681,6 +2681,27 @@ trigger_job:
     strategy: depend
 ```
 
+#### Linking pipelines with `trigger:strategy`
+
+By default, the `trigger` job completes with the `success` status
+as soon as the downstream pipeline is created.
+
+To force the `trigger` job to wait for the downstream (multi-project or child) pipeline to complete, use
+`strategy: depend`. This will make the trigger job wait with a "running" status until the triggered
+pipeline completes. At that point, the `trigger` job will complete and display the same status as
+the downstream job.
+
+```yaml
+trigger_job:
+  trigger:
+    include: path/to/child-pipeline.yml
+    strategy: depend
+```
+
+This can help keep your pipeline execution linear. In the example above, jobs from
+subsequent stages will wait for the triggered pipeline to successfully complete before
+starting, at the cost of reduced parallelization.
+
 ### `interruptible`
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/23464) in GitLab 12.3.