planner: add document about some new introduced hints

[ran-huang]

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• This PR is translated from: planner: add document about some new introduced hints docs-cn#14664
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[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-03 07:47:42.102835731 +0000 UTC m=+344346.045184257: ☑️ agreed by qiancai.

[ran-huang]

/lgtm

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[ran-huang]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #14855.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14857.