perf test: Fix parallel/sequential option documentation

perf test: Fix parallel/sequential option documentation

The parallel option was removed in commit 94d1a913bdc4 ("perf test:
Make parallel testing the default"). Update the sequential
documentation to reflect it isn't the default except for "exclusive"
tests.

Fixes: 94d1a913bdc4 ("perf test: Make parallel testing the default")
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/20250110045736.598281-4-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>