workqueue: Add "Affinity Scopes and Performance" section to documentation

With affinity scopes and their strictness setting added, unbound workqueues
should now be able to cover wide variety of configurations and use cases.
Unfortunately, the performance picture is not entirely straight-forward due
to a trade-off between efficiency and work-conservation in some situations
necessitating manual configuration.

This patch adds "Affinity Scopes and Performance" section to
Documentation/core-api/workqueue.rst which illustrates the trade-off with a
set of experiments and provides some guidelines.

Signed-off-by: Tejun Heo <tj@kernel.org>