Add a few paragraphs on tool and hwmon events.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/Documentation/perf-list.txt | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/perf/Documentation/perf-list.txt b/tools/perf/Documentation/perf-list.txt
index 14621f39b375..d0c65fad419a 100644
--- a/tools/perf/Documentation/perf-list.txt
+++ b/tools/perf/Documentation/perf-list.txt
@@ -243,6 +243,21 @@ For accessing trace point events perf needs to have read access to
 /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
 -------
 
-- 
2.47.0.163.g1226f6d8fa-goog