Improve the documentation on the merge request tracking on deploy

doc/api/deployments.md

@@ -358,6 +358,12 @@ Example response:
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/35739) in GitLab 12.7.
 
+NOTE:
+Not all deployments can be associated with merge requests.
+Please see
+[Track what merge requests were deployed to an environment](../ci/environments/index.md#track-newly-included-merge-requests-per-deployment)
+for more information.
+
 This API retrieves the list of merge requests shipped with a given deployment:
 
 ```plaintext

doc/ci/environments/index.md

@@ -308,6 +308,31 @@ Note the following:
   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).
 
+## Track newly included merge requests per deployment
+
+GitLab can track newly included merge requests per deployment.
+When a deployment succeeded, the system calculates commit-diffs between the latest deployment and the previous deployment.
+This tracking information can be fetched via the [Deployment API](../../api/deployments.md#list-of-merge-requests-associated-with-a-deployment)
+and displayed at a post-merge pipeline in [merge request pages](../../user/project/merge_requests/index.md).
+
+To activate this tracking, your environment must be configured in the following:
+
+- [Environment name](../yaml/index.md#environmentname) is not foldered with `/` (that is, top-level/long-lived environments), _OR_
+- [Environment tier](#deployment-tier-of-environments) is either `production` or `staging`.
+
+Here are the example setups of [`environment` keyword](../yaml/index.md#environment) in `.gitlab-ci.yml`:
+
+```yaml
+# Trackable
+environment: production
+environment: production/aws
+environment: development
+
+# Non Trackable
+environment: review/$CI_COMMIT_REF_SLUG
+environment: testing/aws
+```
+
 ## Working with environments
 
 Once environments are configured, GitLab provides many features for working with them,