What are host reservations, and how to use them?

Request from support for more explanation, in the KB or the ARM for users who don't understand what host reservations are for, how they are different from regular leases, and what their options are for configuring them.

• in the (dynamic) address pool
• global
• in Kea config
• in shared db backend

We also have a request, from a support customer, to explain how Host Reservations in Kea compare to Host reservations in ISC DHCP, in terms of

• default support for global scope reservations
• how clients are matched with host reservations
• whether the same HR/MAC can have multiple IPs
• anything else significant the admin migrating to Kea from ISC DHCP needs to know about HRs