rndc dumpdb output may be larger than expected - documentation update request

Ordinarily it should be anticipated by operators that dumping cache via rndc dumpdb is going to create a large file in the dump location. However there are some RRs whose presentation form can be significantly larger than their wire/memory format.

For example, the NSEC record format contains a bitmap which, when expanded can consume significantly more bytes.

This is a request for the ARM to be updated to warn operators that dumpdb output files can be significantly large - sometimes bigger than the operator would expect if there are many NSEC/NSEC3 RRsets currently cached.