Add comment to specify timeout unit in ConditionVariableTimedSleep()

Hi hackers,

ConditionVariableTimedSleep() accepts a timeout parameter, but it
doesn't explicitly state the unit for the timeout anywhere. To
determine this, one needs to look into the details of the function to
find it out from the comments of the internally called function
WaitLatch(). It would be beneficial to include a comment in the header
of ConditionVariableTimedSleep() specifying that the timeout is in
milliseconds, similar to what we have for other non-static functions
like WaitLatch and WaitEventSetWait. Attached the patch for the same.

thanks
Shveta