doc: watchdog: clarify hardlockup detection timing

doc: watchdog: clarify hardlockup detection timing

The current documentation implies that a hardlockup is strictly defined as
looping for "more than 10 seconds." However, the detection mechanism is
periodic (based on `watchdog_thresh`), meaning detection time varies
significantly depending on when the lockup occurs relative to the NMI perf
event.

Update the definition to remove the strict "more than 10 seconds"
constraint in the introduction and defer details to the Implementation
section.

Additionally, add a "Detection Overhead" section illustrating the Best
Case (~6s) and Worst Case (~20s) detection scenarios to provide
administrators with a clearer understanding of the watchdog's latency.

Link: https://lkml.kernel.org/r/20260312-hardlockup-watchdog-fixes-v2-3-45bd8a0cc7ed@google.com
Signed-off-by: Mayank Rungta <mrungta@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Li Huafei <lihuafei1@huawei.com>
Cc: Max Kellermann <max.kellermann@ionos.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Stephane Erainan <eranian@google.com>
Cc: Wang Jinchao <wangjinchao600@gmail.com>
Cc: Yunhui Cui <cuiyunhui@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>