tidb-lightning: refine the PITR support info

[qiancai]

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[LGTM Timeline notifier]

Timeline:

• 2024-09-25 03:10:57.844496758 +0000 UTC m=+1621927.584920704: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #18740.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18741.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18742.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #18743.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #18744.