Document new arguments in run-tests.sh

Problem/Motivation

In #3386076: GitLab CI integration for core we added two new arguments to the run-tests.sh to allow paralell runs, but the simpletest_script_help() was not updated to describe these new parameters.

diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
@@ -404,6 +404,8 @@ function simpletest_script_parse_args() {
     'xml' => '',
     'non-html' => FALSE,
+    'ci-parallel-node-index' => 1,
+    'ci-parallel-node-total' => 1,
 
@@ -1024,6 +1026,12 @@ function simpletest_script_get_test_list() {
+  if ((int) $args['ci-parallel-node-total'] > 1) {
+    $tests_per_job = ceil(count($test_list) / $args['ci-parallel-node-total']);
+    $test_list = array_slice($test_list, ($args['ci-parallel-node-index'] - 1) * $tests_per_job, $tests_per_job);
+  }

I think we should add at least a brief description here.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet