IPv6 reverse DNS details

[buraglio]

Details around lack of easily usable dynamic reverse DNS is probably missing, KEA is mentioned but there may be some text to add surrounding -PD allocations and the lack of PTRs there.

[becarpenter]

@buraglio, any chance you can suggest some text? It would fit naturally at the end of https://github.com/becarpenter/book6/blob/main/06.%20Management%20and%20Operations/Address%20and%20Prefix%20Management.md