kernel: work_q: Fix k_work_queue_start documentation

Inform that the queue has to be initialized in zeroed memory or with
the k_work_queue_init before use.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
diff --git a/include/kernel.h b/include/kernel.h
index ec03796..7183fee 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -3061,7 +3061,9 @@
  * This configures the work queue thread and starts it running.  The function
  * should not be re-invoked on a queue.
  *
- * @param queue pointer to the queue structure.
+ * @param queue pointer to the queue structure. It must be initialized
+ *        in zeroed/bss memory or with @ref k_work_queue_init before
+ *        use.
  *
  * @param stack pointer to the work thread stack area.
  *