[SPARK-33617][SQL][FOLLOWUP] refine the default parallelism SQL config

[cloud-fan]

What changes were proposed in this pull request?

This is a followup of #30559 . The default parallelism config in Spark core is not good, as it's unclear where it applies. To not inherit this problem in Spark SQL, this PR refines the default parallelism SQL config, to make it clear that it only applies to leaf nodes.

Why are the changes needed?

Make the config clearer.

Does this PR introduce any user-facing change?

It changes an unreleased config.

How was this patch tested?

existing tests

[cloud-fan]

cc @wangyum @dongjoon-hyun @maropu @maryannxue

[SparkQA]

Test build #132662 has finished for PR 30736 at commit 5473140.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Could you fix the build failure?

[SparkQA]

Test build #132810 has finished for PR 30736 at commit f89e266.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #132816 has finished for PR 30736 at commit 2d82349.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/37419/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/37419/

[SparkQA]

Test build #132817 has finished for PR 30736 at commit 7b05413.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

thanks for the review, merging to master!