Add comment about units of backup_keep_time to gitlab.rb.template

https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2333

files/gitlab-config-template/gitlab.rb.template

@@ -232,6 +232,7 @@ external_url 'GENERATED_EXTERNAL_URL'
 # gitlab_rails['backup_archive_permissions'] = 0644
 # gitlab_rails['backup_pg_schema'] = 'public'
+# The units of backup_keep_time are seconds.
 # gitlab_rails['backup_keep_time'] = 604800
 # gitlab_rails['backup_upload_connection'] = {
 #   'provider' => 'AWS',