doc: admin-guide: remove explanation of "watchdog/%u"

"watchdog/%u" threads has be replaced by cpu_stop_work,
which will mislead the reader.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Link: https://lore.kernel.org/r/1615801744-31548-1-git-send-email-wangqing@vivo.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>