doc: watchdog: document buddy detector

The current documentation generalizes the hardlockup detector as primarily
NMI-perf-based and lacks details on the SMP "Buddy" detector.

Update the documentation to add a detailed description of the Buddy
detector, and also restructure the "Implementation" section to explicitly
separate "Softlockup Detector", "Hardlockup Detector (NMI/Perf)", and
"Hardlockup Detector (Buddy)".

Clarify that the softlockup hrtimer acts as the heartbeat generator for
both hardlockup mechanisms and centralize the configuration details in a
"Frequency and Heartbeats" section.

Link: https://lkml.kernel.org/r/20260312-hardlockup-watchdog-fixes-v2-5-45bd8a0cc7ed@google.com
Signed-off-by: Mayank Rungta <mrungta@google.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: Petr Mladek <pmladek@suse.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>

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