perf test: Improve verbose documentation

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

Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250110045736.598281-6-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>