perf test: Fix parallel/sequential option documentation

The parallel option was removed in commit 94d1a91 ("perf test:
Make parallel testing the default"). Update the sequential
documentation to reflect it isn't the default except for "exclusive"
tests.

Fixes: 94d1a91 ("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>

Author: captain5050

tools/perf/Documentation/perf-test.txt

@@ -33,13 +33,9 @@ OPTIONS
 
 -S::
 --sequential::
-	Run tests one after the other, this is the default mode.
-
--p:: 
---parallel::
-	Run tests in parallel, speeds up the whole process but is not safe with
-	the current infrastructure, where some tests that compete for some resources,
-	for instance, 'perf probe' tests that add/remove probes or clean all probes, etc.
+	Run all tests one after the other. By default "exclusive"
+	tests are run sequentially, but other tests are run in
+	parallel to speed execution.
 
 -F::
 --dont-fork::