Merge branch 'docs/subgroups-mysql-ee' into 'master'

Add link to issue for MySQL/subgroups drop support

See merge request !2184

doc/user/group/subgroups/index.md

@@ -2,8 +2,8 @@
 
 >**Notes:**
 - [Introduced][ce-2772] in GitLab 9.0.
-- MySQL support for subgroups was dropped in GitLab 9.3
-  [due to performance reasons][issue].
+- Not available when using MySQL as external database (support removed in
+  GitLab 9.3 [due to performance reasons][issue]).
 
 With subgroups (aka nested groups or hierarchical groups) you can have
 up to 20 levels of nested groups, which among other things can help you to: