Update syncpoint doc

[sdojjy]

First-time contributors' checklist

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add some notes when using async point feature

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• master (the latest development version)
• v7.6 (TiDB 7.6 versions)
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• v7.3 (TiDB 7.3 versions)
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[flowbehappy]

LGTM

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[qiancai]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #16571.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.4: #16572.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #16573.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #16574.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #16575.