Update follower read and learner read descriptions (#21470)

[ti-chi-bot]

This is an automated cherry-pick of #21470

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• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

The description of follower-read and learner-read is inaccurate, and related behaviors have been adjusted in the newer version of tidb, so the corresponding documentation has been revised accordingly.

Which TiDB version(s) do your changes apply to? (Required)

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For details, see tips for choosing the affected versions.

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: Update follower read and learner read descriptions docs-cn#20609
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the documentation for TiDB's follower-read and learner-read functionalities. The changes aim to correct inaccuracies and reflect adjusted behaviors in newer TiDB versions, providing clearer descriptions of how tidb_replica_read operates when configured for follower or learner modes, including fallback mechanisms and version-specific considerations.

Highlights

• Updated follower read behavior description: The documentation for tidb_replica_read set to follower now clarifies that learner replicas are also considered for reads, and if no follower or learner replicas are available, TiDB falls back to reading from the leader.
• Updated learner read behavior description: The documentation for tidb_replica_read set to learner now specifies that if no learner replica is available, TiDB will read from an available leader or follower replica.
• Added version-specific notes: A new note has been added to clarify the behavior for TiDB v7.5.0 and earlier versions, where setting tidb_replica_read to follower or learner would result in an error if no appropriate replicas were available.

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[gemini-code-assist]

Code Review

This pull request updates the documentation for follower-read and learner-read to reflect recent behavior changes in TiDB. The changes clarify that when a follower or learner replica is not available, TiDB will fall back to reading from another available replica instead of returning an error. The documentation for older versions is also updated to reflect the previous behavior. My review includes a few suggestions to improve clarity and conciseness.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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