Documentation for the ip-reservations-unique feature

This MR is going to cover ARM documentation for the new feature which allows for creating multiple IP reservations in the same subnet. Whether this is allowed or not is controlled via a new global parameter ip-reservations-unique. We need to document this parameter, describe which backends support this setting and what happens if they don't. We also have to mention the case of configuring it via Config Backend and what happens when it is unsuccessful. We need to describe the case when this setting is flipped from false to true and some duplicated reservations possibly exist.