pd: add rank-formula-version config for hot scheduler

[HunDunDM]

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What is changed, added or deleted? (Required)

Add description to the rank-formula-version config of PD's balance-hot-region-scheduler.

Related PD PR: pd#5515

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[Oreoxmt]

/status LGT1

[Oreoxmt]

LGTM

[shichun-0415]

@HunDunDM Please help add the experimental feature to the experimental features file. Thanks.

[Oreoxmt]

/merge

[ti-chi-bot]

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

Commit hash: 903ef35

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11671.