[9.18] fix: doc: Clarify dnssec-signzone interval option

There was confusion about whether the interval was calculated from
the validity period provided on the command line (with -s and -e),
or from the signature being replaced.

Add text to clarify that the interval is calculated from the new
validity period.

Closes #5128

Backport of MR !9955

Merge branch 'backport-5128-clarify-dnssec-signzone-interval-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9984