doc: kernel: workqueue: explicitly mention K_WORK_DEFINE
doc: kernel: workqueue: explicitly mention K_WORK_DEFINE

Point out that this macro can be used to avoid the need to initialize
a work item.  This is still of limited use since it can't statically
initialize a work item within another structure.

Signed-off-by: Peter Bigot <a4112a160b558d5427af084318038e19a95979ee@nordicsemi.no>

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