perf docs: Document tool and hwmon events

Add a few paragraphs on tool and hwmon events.

Signed-off-by: Ian Rogers <[e-mail]>
Acked-by: Namhyung Kim <[e-mail]>
Cc: Ravi Bangoria <[e-mail]>
Cc: Yoshihiro Furudera <[e-mail]>
Cc: Howard Chu <[e-mail]>
Cc: Ze Gao <[e-mail]>
Cc: Changbin Du <[e-mail]>
Cc: Junhao He <[e-mail]>
Cc: Weilin Wang <[e-mail]>
Cc: James Clark <[e-mail]>
Cc: Oliver Upton <[e-mail]>
Cc: Athira Jajeev <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/Documentation/perf-list.txt

···
 /sys/kernel/tracing, even when perf_event_paranoid is in a relaxed
 setting.
 
+TOOL/HWMON EVENTS
+-----------------
+
+Some events don't have an associated PMU instead reading values
+available to software without perf_event_open. As these events don't
+support sampling they can only really be read by tools like perf stat.
+
+Tool events provide times and certain system parameters. Examples
+include duration_time, user_time, system_time and num_cpus_online.
+
+Hwmon events provide easy access to hwmon sysfs data typically in
+/sys/class/hwmon. This information includes temperatures, fan speeds
+and energy usage.
+
+
 TRACING
 -------