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

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 <[email protected]>
Reviewed-by: Tejun Heo <[email protected]>
Acked-by: Frederic Weisbecker <[email protected]>
Reviewed-by: Lai Jiangshan <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>