watchdog: Remove softlockup_thresh from Documentation

The old softlockup detector has been replaced with new lockup
detector long ago.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Link: http://lkml.kernel.org/r/51959687.9090305@huawei.com


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