perf stat: Document --metric-no-threshold and threshold colors

Document the threshold behavior for -M/--metrics.
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ahmad Yasin <ahmad.yasin@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Caleb Biggers <caleb.biggers@intel.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Edward Baker <edward.baker@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Perry Taylor <perry.taylor@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Samantha Alt <samantha.alt@intel.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Link: https://lore.kernel.org/r/20230519063719.1029596-2-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -361,6 +361,15 @@ small group that need not have multiplexing is lowered. This option
 forbids the event merging logic from sharing events between groups and
 may be used to increase accuracy in this case.
 
+--metric-no-threshold::
+Metric thresholds may increase the number of events necessary to
+compute whether a metric has exceeded its threshold expression. This
+may not be desirable, for example, as the events can introduce
+multiplexing. This option disables the adding of threshold expression
+events for a metric. However, if there are sufficient events to
+compute the threshold then the threshold is still computed and used to
+color the metric's computed value.
+
 --quiet::
 Don't print output, warnings or messages. This is useful with perf stat
 record below to only write data to the perf.data file.
@@ -405,6 +414,12 @@ For a group all metrics from the group are added.
 The events from the metrics are automatically measured.
 See perf list output for the possible metrics and metricgroups.
 
+	When threshold information is available for a metric, the
+	color red is used to signify a metric has exceeded a threshold
+	while green shows it hasn't. The default color means that
+	no threshold information was available or the threshold
+	couldn't be computed.
+
 -A::
 --no-aggr::
 Do not aggregate counts across all monitored CPUs.