watchdog: Add comments to explain the watchdog_disabled variable

The 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.

Signed-off-by: anish kumar <anish198519851985@gmail.com>
[small text cleanups]
Signed-off-by: Don Zickus <dzickus@redhat.com>
Cc: chuansheng.liu@intel.com
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1363113848-18344-1-git-send-email-dzickus@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>