Add link to issue for MySQL/subgroups drop support

doc/install/requirements.md

@@ -100,7 +100,7 @@ We currently support the following databases:
 We **highly recommend** the use of PostgreSQL instead of MySQL/MariaDB as not all
 features of GitLab may work with MySQL/MariaDB:
 
-1. MySQL support for subgroups was dropped with GitLab 9.3.
+1. MySQL support for subgroups was [dropped with GitLab 9.3][post].
    See [issue #30472][30472] for more information.
 1. GitLab Geo does [not support MySQL](https://docs.gitlab.com/ee/gitlab-geo/database.html#mysql-replication).
 1. [Zero downtime migrations][zero] do not work with MySQL
@@ -110,6 +110,7 @@ Existing users using GitLab with MySQL/MariaDB are advised to
 
 [30472]: https://gitlab.com/gitlab-org/gitlab-ce/issues/30472
 [zero]: ../update/README.md#upgrading-without-downtime
+[post]: https://about.gitlab.com/2017/06/22/gitlab-9-3-released/#dropping-support-for-subgroups-in-mysql
 
 ### PostgreSQL Requirements
 

doc/user/group/subgroups/index.md

@@ -2,7 +2,8 @@
 
 >**Notes:**
 - [Introduced][ce-2772] in GitLab 9.0.
-- MySQL support for subgroups was dropped in GitLab 9.3.
+- 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:
@@ -175,3 +176,4 @@ Here's a list of what you can't do with subgroups:
 [ce-2772]: https://gitlab.com/gitlab-org/gitlab-ce/issues/2772
 [permissions]: ../../permissions.md#group
 [reserved]:  https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/path_regex.rb
+[issue]: https://gitlab.com/gitlab-org/gitlab-ce/issues/30472#note_27747600