genirq/irqreturn: Fix kernel-doc warnings

irqreturn.h:6: warning: missing initial short description on line:
 * enum irqreturn
irqreturn.h:15: warning: Enum value 'IRQ_NONE' not described in enum 'irqreturn'
irqreturn.h:15: warning: Enum value 'IRQ_HANDLED' not described in enum 'irqreturn'
irqreturn.h:15: warning: Enum value 'IRQ_WAKE_THREAD' not described in enum 'irqreturn'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221124063013.28479-1-rdunlap@infradead.org