kthread_worker: document CPU hotplug handling

The kthread worker API is simple.  In short, it allows to create, use, and
destroy workers.  kthread_create_worker_on_cpu() just allows to bind a
newly created worker to a given CPU.

It is up to the API user how to handle CPU hotplug.  They have to decide
how to handle pending work items, prevent queuing new ones, and restore
the functionality when the CPU goes off and on.  There are few catches:

   + The CPU affinity gets lost when it is scheduled on an offline CPU.

   + The worker might not exist when the CPU was off when the user
     created the workers.

A good practice is to implement two CPU hotplug callbacks and
destroy/create the worker when CPU goes down/up.

Mention this in the function description.

[akpm@linux-foundation.org: grammar tweaks]

Link: https://lore.kernel.org/r/20201028073031.4536-1-qiang.zhang@windriver.com
Link: https://lkml.kernel.org/r/20201102101039.19227-1-pmladek@suse.com
Reported-by: Zhang Qiang <Qiang.Zhang@windriver.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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