kernel/watchdog.c: add comments to explain watchdog_disabled variable

kernel/watchdog.c: add comments to explain watchdog_disabled variable

This watchdog_disabled flag is a bit cryptic.  However it's usefulness is
multifold.  Uses are:

1. Check if smpboot_register_percpu_thread function passed.
2. Makes sure that user enables and disables the watchdog in sequence
   i.e. enable watchdog->disable watchdog->enable watchdog
   Unlike enable watchdog->enable watchdog which is wrong.

[dzickus@redhat.com: small text cleanups]
Signed-off-by: anish kumar <anish198519851985@gmail.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>