genirq: Add missing documentation for tot_count

Commit:

  1136b072 ("genirq: Avoid summation loops for /proc/stat")

adds a new irq_desc::tot_count field, without documenting it.
Add the missing piece of documentation.
Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Daniel Colascione <dancol@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1549983253-19107-1-git-send-email-longman@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/irqdesc.h

@@ -28,6 +28,7 @@ struct pt_regs;
  * @core_internal_state__do_not_mess_with_it: core internal status information
  * @depth:		disable-depth, for nested irq_disable() calls
  * @wake_depth:		enable depth, for multiple irq_set_irq_wake() callers
+ * @tot_count:		stats field for non-percpu irqs
  * @irq_count:		stats field to detect stalled irqs
  * @last_unhandled:	aging timer for unhandled count
  * @irqs_unhandled:	stats field for spurious unhandled interrupts