Improve documentation about pipeline configuration for review apps

This commit improves the documentation.

doc/ci/environments/index.md

@@ -219,10 +219,17 @@ You can also specify a static part of the URL at `environment:url:`, such as
 
 The assigned URL for the `review/your-branch-name` environment is [visible in the UI](#using-the-environment-url).
 
-> **Notes:**
->
-> - `stop_review` doesn't generate a dotenv report artifact, so it won't recognize the `DYNAMIC_ENVIRONMENT_URL` variable. Therefore you should not set `environment:url:` in the `stop_review` job.
-> - If the environment URL is not valid (for example, the URL is malformed), the system doesn't update the environment URL.
+Note the following:
+
+- `stop_review` doesn't generate a dotenv report artifact, so it won't recognize the
+  `DYNAMIC_ENVIRONMENT_URL` variable. Therefore you shouldn't set `environment:url:` in the
+  `stop_review` job.
+- If the environment URL isn't valid (for example, the URL is malformed), the system doesn't update
+  the environment URL.
+- If the script that runs in `stop_review` exists only in your repository and therefore can't use
+  `GIT_STRATEGY: none`, configure [pipelines for merge requests](../../ci/merge_request_pipelines/index.md)
+  for these jobs. This ensures that runners can fetch the repository even after a feature branch is
+  deleted. For more information, see [Ref Specs for Runners](../pipelines/index.md#ref-specs-for-runners).
 
 ### Configuring manual deployments
 
@@ -675,24 +682,23 @@ deploy_review:
     name: review/$CI_COMMIT_REF_NAME
     url: https://$CI_ENVIRONMENT_SLUG.example.com
     on_stop: stop_review
-  only:
-    - branches
-  except:
-    - master
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
 
 stop_review:
   stage: deploy
-  variables:
-    GIT_STRATEGY: none
   script:
     - echo "Remove review app"
-  when: manual
   environment:
     name: review/$CI_COMMIT_REF_NAME
     action: stop
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
+      when: manual
 ```
 
-Setting the [`GIT_STRATEGY`](../yaml/README.md#git-strategy) to `none` is necessary in the
+If you can't use [Pipelines for merge requests](../merge_request_pipelines/index.md),
+setting the [`GIT_STRATEGY`](../yaml/README.md#git-strategy) to `none` is necessary in the
 `stop_review` job so that the [runner](https://docs.gitlab.com/runner/) won't
 try to check out the code after the branch is deleted.
 
@@ -748,12 +754,16 @@ review_app:
     name: review/$CI_COMMIT_REF_NAME
     on_stop: stop_review_app
     auto_stop_in: 1 week
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
 
 stop_review_app:
   script: stop-review-app
   environment:
     name: review/$CI_COMMIT_REF_NAME
     action: stop
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
       when: manual
 ```
 

doc/ci/pipelines/index.md

@@ -79,6 +79,27 @@ You can also configure specific aspects of your pipelines through the GitLab UI.
 - [Pipeline schedules](schedules.md).
 - [Custom CI/CD variables](../variables/README.md#custom-environment-variables).
 
+### Ref Specs for Runners
+
+When a runner picks a pipeline job, GitLab provides that job's metadata. This includes the [Git refspecs](https://git-scm.com/book/en/v2/Git-Internals-The-Refspec),
+which indicate which ref (branch, tag, and so on) and commit (SHA1) are checked out from your
+project repository.
+
+This table lists the refspecs injected for each pipeline type:
+
+| Pipeline type                                                      | Refspecs                                                                                       |
+|---------------                                                     |----------------------------------------                                                        |
+| Pipeline for Branches                                              | `+refs/pipelines/<id>:refs/pipelines/<id>` and `+refs/heads/<name>:refs/remotes/origin/<name>` |
+| pipeline for Tags                                                  | `+refs/pipelines/<id>:refs/pipelines/<id>` and `+refs/tags/<name>:refs/tags/<name>`            |
+| [Pipeline for Merge Requests](../merge_request_pipelines/index.md) | `+refs/pipelines/<id>:refs/pipelines/<id>`                                                     |
+
+The refs `refs/heads/<name>` and `refs/tags/<name>` exist in your
+project repository. GitLab generates the special ref `refs/pipelines/<id>` during a
+running pipeline job. This ref can be created even after the associated branch or tag has been
+deleted. It's therefore useful in some features such as [automatically stopping an environment](../environments/index.md#automatically-stopping-an-environment),
+and [merge trains](../merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md)
+that might run pipelines after branch deletion.
+
 ### View pipelines
 
 You can find the current and historical pipeline runs under your project's