While working on #19444, I assumed that omitting `Start` would return a
clock that started at January 1, year 1, because that's the zero value
for a `time.Time`, but actually it uses the current UTC time instead.
This behaviour is non-obvious, so document it.
Updates #cleanup
Change-Id: Id91400778578655953ff3e1671ce470db97cfe91
Signed-off-by: Alex Chan <alexc@tailscale.com>
Alex Chan
10b63f27ce