workqueue: doc: Call out the non-reentrance conditions

The current doc of workqueue API suggests that work items are
non-reentrant: any work item is guaranteed to be executed by at most one
worker system-wide at any given time. However this is not true, the
following case can cause a work item W executed by two workers at
the same time:

        queue_work_on(0, WQ1, W);
        // after a worker picks up W and clear the pending bit
        queue_work_on(1, WQ2, W);
        // workers on CPU0 and CPU1 will execute W in the same time.

, which means the non-reentrance of a work item is conditional, and
Lai Jiangshan provided a nice summary[1] of the conditions, therefore
use it to describe a work item instance and improve the doc.

[1]: https://lore.kernel.org/lkml/CAJhGHyDudet_xyNk=8xnuO2==o-u06s0E0GZVP4Q67nmQ84Ceg@mail.gmail.com/

Suggested-by: Matthew Wilcox <willy@infradead.org>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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