[SQL] some comments fix for GROUPING SETS

[adrian-wang]

No description provided.

[SparkQA]

Test build #25401 has started for PR 4000 at commit 9c24fc4.

• This patch merges cleanly.

[SparkQA]

Test build #25401 has finished for PR 4000 at commit 9c24fc4.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[adrian-wang]

retest this please.

[SparkQA]

Test build #25408 has started for PR 4000 at commit 9c24fc4.

• This patch merges cleanly.

[SparkQA]

Test build #25408 has finished for PR 4000 at commit 9c24fc4.

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

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[yhuai]

LGTM

[rxin]

Merging in master. Thanks.

[rxin]

BTW this is the 4000 PR :)