Document update (hooks for CA)

From: https://kea.readthedocs.io/en/kea-2.3.4/arm/hooks.html#available-hook-libraries

Warning
While the Kea Control Agent includes the "hooks" functionality, (i.e. hook libraries can be loaded by this process), none of ISC's current hook libraries should be loaded by the Control Agent.

This is no longer correct - the RBAC hooks libraries are loaded on the CA