planner: add some comment for checkOnlyFullGroupBy

[xuyifangreeneyes]

What problem does this PR solve?

see #23844 (comment)

What is changed and how it works?

add comment for #23844

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test

Release note

• add some comments for checkOnlyFullGroupBy

[xuyifangreeneyes]

/cc @Reminiscent

[xuyifangreeneyes]

/cc @winoros

[xuyifangreeneyes]

/sig planner

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Reminiscent
• winoros

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[Reminiscent]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 20622fa

[ti-chi-bot]

@xuyifangreeneyes: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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