kern_ntptime: document handling of non-positive time constant

[go4av05.pvtx_gmail.com]

Summary

Clarify that zero or negative time constant values supplied via
ntp_adjtime(2) are intentionally ignored to preserve stable clock
discipline.

No functional change.

Test Plan

Not applicable (comment-only change).

[imp]

sys/kern/kern_ntptime.c

394

No.

This is the UTC / TAI offset. It's always going to be positive because you can't have enough negative leap seconds to undo the 37 leap seconds we've had since 1972 (in fact, we've never had one, and we're also going to eliminate them here in the next few years). Even at the worst rate of negative slope, it would take 300 years to accumulate this many leap seconds.

It has to be positive, because we've only ever had positive leap seconds. It has nothing to do with clock discipline (eg keeping the timebase ticking properly).