Do not flush the cache for `rndc validation status`

Do not flush the cache for `rndc validation status`

And add a note to the man page that `rndc validation` flushes the
cache when the validation state is changed. (It is necessary to flush
the cache when turning on validation, to avoid continuing to use
cryptographically invalid data. It is probably wise to flush the cache
when turning off validation to recover from lameness problems.)