Add note that logged intervals are always in seconds

[timwoj]

From https://zeekorg.slack.com/archives/CSZBXF6TH/p1741284694941779:

Aaron J. Scantlin
i couldn’t find a definitive answer in the docs (in either connection docs or docs on the interval type), but: in conn.log, i assume the duration is in seconds?

Vern Paxson
correct
when an interval type is print’d, it has modifiers, like 5.0 mins. But for logs, this is always rendered as a double representing seconds