Adding note on migrations to Gitlab Docs

doc/integration/elasticsearch.md

@@ -435,6 +435,55 @@ After the reindexing is completed, the original index will be scheduled to be de
 
 While the reindexing is running, you will be able to follow its progress under that same section.
 
+## Background migrations
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/234046) in GitLab 13.6.
+
+With reindex migrations running in the background, there's no need for a manual
+intervention. This usually happens in situations where new features are added to
+Advanced Search, which means adding or changing the way content is indexed.
+
+To confirm that the background migrations ran, you can check with:
+
+```shell
+curl "$CLUSTER_URL/gitlab-production-migrations/_search?q=*" | jq .
+```
+
+This should return something similar to:
+
+```json
+{
+  "took": 14,
+  "timed_out": false,
+  "_shards": {
+    "total": 1,
+    "successful": 1,
+    "skipped": 0,
+    "failed": 0
+  },
+  "hits": {
+    "total": {
+      "value": 1,
+      "relation": "eq"
+    },
+    "max_score": 1,
+    "hits": [
+      {
+        "_index": "gitlab-production-migrations",
+        "_type": "_doc",
+        "_id": "20201105181100",
+        "_score": 1,
+        "_source": {
+          "completed": true
+        }
+      }
+    ]
+  }
+}
+```
+
+In order to debug issues with the migrations you can check the [`elasticsearch.log` file](../administration/logs.md#elasticsearchlog).
+
 ## GitLab Advanced Search Rake tasks
 
 Rake tasks are available to: