Add note to migration docs on setgid change

This commit adds a note to the migration docs for 5.5 that Elasticsearch
sets the setgid flag on /etc/elasticsearch to ensure correct ownership
of files created there.

Author: jasontedor

docs/reference/migration/migrate_5_5.asciidoc

@@ -20,6 +20,15 @@ writing. Starting in Elasticsearch 5.5.0, this flag has been removed and
 instead the same functionality can be achieved by masking the `systemd-sysctl`
 service.
 
+[float]
+=== RPM and Debian packages `setgid` on `/etc/elasticsearch`
+
+The RPM and Debian packages now set the `setgid` flag on `/etc/elasticsearch` so
+that any files and subdirectories created under `/etc/elasticsearch` have the
+same ownership as `/etc/elasticsearch` (`root:elasticsearch`). This is used to
+ensure that files and subdirectories created under `/etc/elasticsearch` (e.g., a
+keystore) are readable by the `elasticsearch` user.
+
 [[breaking_55_rest_changes]]
 [float]
 === 404s on get alias and get mapping APIs