[9.20] fix: doc: Restore text about sig validity and SOA expire

When `sig-validity-interval` was obsoleted, the text that the signature validity interval should be multiples of the SOA expire interval was removed. Restore this text to the description of the `signatures-validity` option.

Closes #4951

Backport of MR !9566

Merge branch 'backport-4951-document-signatures-validity-soa-expire-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9573