document overlapping pools and subnets

Closes #1842 (closed).

Could not find a good comment to put on the entry for PD pools out of subnets, to elegantly explain why they are allowed. Barbarically put, the clients benefitting from the delegated prefixes don't need to be in the same subnet, right?

Additional actions which have a chance of being affected, that I haven't tested, but that I hope we can exclude theoretically:

• HA lease syncing
• leasing, releasing, reclaiming, and leasing again
• maybe also any problems that arise from changing the subnet ID of a subnet?