Update sig-validity-interval description
requested to merge 2890-update-documentation-with-respect-to-sig-validity-interval-and-update-v9_16 into v9_16
Document that the interval on new RRSIG records is randomally chosen between the limits specified by sig-validity-interval. document the operatations when this occurs.
(cherry picked from commit e65ce00f)
Closes #2890 (closed)