mention possibility for reducing scanning frequency

for parents with existing scanning (they can reduce for children known to send NOTIFY)
- for other children, this would be a backwards-incompatible change
and for parents who introduce CDS/CDNSKEY support for the first time; can use low-frequency scanning for all children
- as the scanning is new, there is no backwards-compatibility problem

peterthomassen / draft-ietf-dnsop-generalized-notify