Documentation/kernel-per-CPU-kthreads.txt: Workqueue affinity

This commit documents the ability to apply CPU affinity to WQ_SYSFS
workqueues, thus offloading them from the desired worker CPUs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Tejun Heo <tj@kernel.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>