Merge branch 'fix-soft-deletion-docs' into 'master'

Document `marked_for_deletion_on` attribute in Groups API response

See merge request gitlab-org/gitlab!33478

doc/api/groups.md

@@ -578,6 +578,18 @@ Additional response parameters:
 }
 ```
 
+Users on GitLab [Silver, Premium, or higher](https://about.gitlab.com/pricing/) will also see
+the `marked_for_deletion_on` attribute:
+
+```json
+{
+  "id": 4,
+  "description": "Aliquid qui quis dignissimos distinctio ut commodi voluptas est.",
+  "marked_for_deletion_on": "2020-04-03",
+  ...
+}
+```
+
 When adding the parameter `with_projects=false`, projects will not be returned.
 
 ```shell