Add DIY use of CB schema is not recommended text (see 1.6.0 Release Notes sec 2) to ARM

We have another user/issue stemming from manual use of CB and subsequent "wrong" behavior. We need to add clear text in Section 5.2 "Kea Configuration Backend" similar to what is in 1.6.0 release notes

Management commands for the Config Backend. The CB stores data in a MySQL schema that is public. It's possible to insert a configuration into the MySQL tables manually, or automatically using SQL scripts, but this requires a reasonably good knowledge of the schema. As an alternative, ISC has developed a new hooks library that provides management commands for config backends. It simplifies many typical operations, such as listing, adding, retrieving, and deleting of global parameters, shared networks, subnets, pools, options, and option definitions. For a complete list, see commands starting with "remote-" in Appendix A of the Kea Administrator Reference Manual. The cb_cmds hooks library is available to subscribers only. If you are not a subscriber and would like to subscribe, please contact info@isc.org and our sales team will assist you.

I suggest we had some additional language excerpted from Vicky's response on kea-users:

The schema for the configuration backend is documented in the code, here: https://gitlab.isc.org/isc-projects/kea/blob/master/src/share/database/scripts/mysql/dhcpdb_create.mysql  We also have some related design documents in our public Gitlab - https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design. 

So, a do-it-yourselfer can certainly populate the tables directly or develop their own system for modifying the tables. This is not recommended because, as Thomas pointed out, it is easy to screw things up directly modifying the database.  But, the feature enabling the configuration backend is open source, and you are welcome to modify the open source however you like for your deployment. 

We did just do a webinar on using the subscription CB-cmds hook library to manage the config backend. The recording is on the ISC channel at https://www.youtube.com/watch?v=gnVEO4ThE10. This is the way we recommend you manage the config backend.

Currently, Section 5.2 almost positions manual use of it as expected/normal.