What is the precision arg for callout_reset_sbt() and friends?

[Ian Lepore]

I can't figure out from the code or the manpage what should be passed as
the precision argument to the sleep-related functions that take sbintime
values.  It seems like most existing code passes 0, what effect does
that have?  What should I pass if precision really matters?  What if it
doesn't matter at all?

Most of the time I want something like a 1ms timeout where it really
isn't critical if it turns into 2 or even 5ms.  Other times when I say
1ms I really want something close to 1ms.

-- Ian