Document when Groups API `/subgroups` endpoint introduced

doc/api/groups.md

@@ -82,6 +82,8 @@ GET /groups?custom_attributes[key]=value&custom_attributes[other_key]=other_valu
 
 ## List a groups's subgroups
 
+> [Introduced][ce-15142] in GitLab 10.3.
+
 Get a list of visible direct subgroups in this group.
 When accessed without authentication, only public groups are returned.
 
@@ -513,3 +515,5 @@ And to switch pages add:
 ```
 /groups?per_page=100&page=2
 ```
+
+[ce-15142]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15142