perf test: Improve verbose documentation

Add a little more detail on the output expectations for each verbose
level.

Signed-off-by: Ian Rogers <[e-mail]>
Reviewed-by: Namhyung Kim <[e-mail]>
Cc: James Clark <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/Documentation/perf-test.txt

···
 	Tests to skip (comma separated numeric list).
 
 -v::
+-vv::
+-vvv::
 --verbose::
-	Be more verbose.
+	With a single '-v', verbose level 1, only failing test output
+	is displayed. With '-vv' and higher all test output is shown.
 
 -S::
 --sequential::