genirq: Fix the kerneldoc comment for struct irq_affinity_desc

A recent commit added a new field but did not update the kerneldoc comment,
leading to this build warning:

  ./include/linux/interrupt.h:268: warning: Function parameter or member 'is_managed' not described in 'irq_affinity_desc'

Add the missing information, making the docs build 0.001% quieter.

Fixes: c410abb ("genirq/affinity: Add is_managed to struct irq_affinity_desc")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Dou Liyang <douliyangs@gmail.com>
Link: https://lkml.kernel.org/r/20190108170432.59bae8a6@lwn.net

include/linux/interrupt.h

@@ -260,6 +260,7 @@ struct irq_affinity {
 /**
  * struct irq_affinity_desc - Interrupt affinity descriptor
  * @mask:	cpumask to hold the affinity assignment
+ * @is_managed: 1 if the interrupt is managed internally
  */
 struct irq_affinity_desc {
 	struct cpumask	mask;