perf script: Improve man page description of metrics

Clarify that a metric is based on events, not referring to itself. Also
some improvements with the sentences.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20190711181922.18765-3-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-script.txt

@@ -228,11 +228,11 @@ OPTIONS
 
 	With the metric option perf script can compute metrics for
 	sampling periods, similar to perf stat. This requires
-	specifying a group with multiple metrics with the :S option
+	specifying a group with multiple events defining metrics with the :S option
 	for perf record. perf will sample on the first event, and
-	compute metrics for all the events in the group. Please note
+	print computed metrics for all the events in the group. Please note
 	that the metric computed is averaged over the whole sampling
-	period, not just for the sample point.
+	period (since the last sample), not just for the sample point.
 
 	For sample events it's possible to display misc field with -F +misc option,
 	following letters are displayed for each bit: