Host Reservations as Basic Access Control examples mistakes

In Host Reservations as Basic Access Control examples in ARM, code is not working correctly.

"reservations-in-subnet": true should be changed to "reservation-mode": "global"

In the second example for v4 and v6, it is required for class options to take priority over subnet options, but Kea works the other way around.
Code needs require-client-classes, but the function does not work correctly - see #2863