Update Kea ARM to be more specific about whether or not it's OK to configure overlapping PD pools for v6 subnets to share

Currently it is possible to configure overlapping PD pools between v6 subnets (or even use the same exact PD pool entirely for all subnets).

This 'appears' to work OK. Our review of the situation however was that without more extensive testing and research, we couldn't say 'this is supported' because what we have here is something that is essentially a happy accident. It looks like it does probably work - but only because the check for whether a PD is already allocated, doesn't check the subnet ID!

This request is a simple one for the ARM to clarify that we don't (currently) support overlapping PD pools.