kern: clarify boot time

In FreeBSD, the current time is computed from uptime + boottime. Uptime
is a continuous, smooth function that's monotonically increasing. To
effect changes to the current time, boottime is adjusted. boottime is
mutable and shouldn't be cached against future need. Document the
current implementation, with the caveat that we may stop stepping
boottime on resume in the future and will step uptime instead (noted in
the commit message, but not in the code).

Sponsored by: Netflix
Reviewed by: phk, rpokala
Differential Revision: https://reviews.freebsd.org/D30116