Documentation: core-api: minor workqueue.rst cleanups

Clean up workqueue.rst:
- fix minor typos
- put '@' after `` instead of preceding them (one place)
- use "CPU" instead of "cpu" in text consistently
- quote one function name

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Florian Mickler <florian@mickler.org>
Signed-off-by: Tejun Heo <tj@kernel.org>