Link to a case where we can't support MySQL

This updates the requirements guide to include a link to an issue where
we discuss dropping MySQL support for nested groups. This should give
users a better understanding of why we highly recommend them to use
PostgreSQL instead.

[ci skip]

doc/install/requirements.md

@@ -126,8 +126,11 @@ We currently support the following databases:
 - MySQL/MariaDB
 
 We _highly_ recommend the use of PostgreSQL instead of MySQL/MariaDB as not all
-features of GitLab may work with MySQL/MariaDB. Existing users using GitLab with
-MySQL/MariaDB are advised to migrate to PostgreSQL instead.
+features of GitLab may work with MySQL/MariaDB. For example, MySQL does not have
+the right features to support nested groups in an efficient manner; see
+<https://gitlab.com/gitlab-org/gitlab-ce/issues/30472> for more information
+about this. Existing users using GitLab with MySQL/MariaDB are advised to
+migrate to PostgreSQL instead.
 
 The server running the database should have _at least_ 5-10 GB of storage
 available, though the exact requirements depend on the size of the GitLab