AAP-36934 added note on external databases (#2685)

Author: emurtoug

downstream/modules/platform/ref-guidelines-hosts-groups.adoc

@@ -6,6 +6,10 @@
 * When using an external database, ensure the `[database]` sections of your inventory file are properly set up.
 * To improve performance, do not colocate the database and the {ControllerName} on the same server.
 
+[IMPORTANT]
+====
+When using an external database with {PlatformNameShort}, you must create and maintain that database. Ensure that you clear your external database when uninstalling the {PlatformNameShort}.
+====
 
 .{HubNameStart}
 * If there is an `[automationhub]` group, you must include the variables `automationhub_pg_host` and `automationhub_pg_port`.