Document workaround for `released_at` in `.gitlab-ci.yml`

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/260424

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44700

doc/ci/yaml/README.md

@@ -3969,7 +3969,11 @@ The title of each milestone the release is associated with.
 #### `release:released_at`
 
 The date and time when the release is ready. Defaults to the current date and time if not
-defined. Expected in ISO 8601 format (2019-03-15T08:00:00Z).
+defined. Should be enclosed in quotes and expressed in ISO 8601 format.
+
+```json
+released_at: '2021-03-15T08:00:00Z'
+```
 
 #### Complete example for `release`