kthread: Add kworker kthreads to OS-jitter documentation

The kworker workqueue kthreads can also contribute to OS jitter.
The amount of jitter depends on their use, so this commit adds
documentation on avoiding OS jitter due to workqueue use.

Reported-by: Jonathan Clairembault <jonathan.clairembault@novasparks.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>