irq: update genericirq book location

This book got converted from DocBook. Update its references to
point to the current location.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

kernel/irq/chip.c

@@ -7,7 +7,7 @@
  * This file contains the core interrupt handling code, for irq-chip
  * based architectures.
  *
- * Detailed information is available in Documentation/DocBook/genericirq
+ * Detailed information is available in Documentation/core-api/genericirq.rst
  */
 
 #include <linux/irq.h>

kernel/irq/handle.c

@@ -6,7 +6,7 @@
  *
  * This file contains the core interrupt handling code.
  *
- * Detailed information is available in Documentation/DocBook/genericirq
+ * Detailed information is available in Documentation/core-api/genericirq.rst
  *
  */
 

kernel/irq/irqdesc.c

@@ -4,7 +4,7 @@
  *
  * This file contains the interrupt descriptor management code
  *
- * Detailed information is available in Documentation/DocBook/genericirq
+ * Detailed information is available in Documentation/core-api/genericirq.rst
  *
  */
 #include <linux/irq.h>