docs: Add infoschema changes to Placement Rules in SQL

[morgo]

What problem does this PR solve?

Problem Summary:

This makes changes to #18030 based on discussions.

What is changed and how it works?

What's Changed:

Updates to infoschema plan.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lcwangchao
• mjonss

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[mjonss]

LGTM

[lcwangchao]

/merge

[ti-chi-bot]

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

Commit hash: 5794ebe

[ti-chi-bot]

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