doc: watchdog: modify the doc related to "watchdog/%u"

"watchdog/%u" threads has be replaced by cpu_stop_work. The current
description is extremely misleading.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Wang Qing <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Cc: "Guilherme G. Piccoli" <[e-mail]>
Cc: Joe Perches <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Mauro Carvalho Chehab <[e-mail]>
Cc: Qais Yousef <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Santosh Sivaraj <[e-mail]>
Cc: Stephen Kitt <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/admin-guide/sysctl/kernel.rst

···
 = =================================
 
 The soft lockup detector monitors CPUs for threads that are hogging the CPUs
-without rescheduling voluntarily, and thus prevent the 'watchdog/N' threads
-from running. The mechanism depends on the CPUs ability to respond to timer
-interrupts which are needed for the 'watchdog/N' threads to be woken up by
-the watchdog timer function, otherwise the NMI watchdog — if enabled — can
-detect a hard lockup condition.
+without rescheduling voluntarily, and thus prevent the 'migration/N' threads
+from running, causing the watchdog work fail to execute. The mechanism depends
+on the CPUs ability to respond to timer interrupts which are needed for the
+watchdog work to be queued by the watchdog timer function, otherwise the NMI
+watchdog — if enabled — can detect a hard lockup condition.
 
 
 stack_erasing