Clarify ReplicaReadMixed also uses learner replicas

[King-Dylan]

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Close issue 22169
Currently, the official docs say these modes will try leader and follower replicas only.

However, from the KV client implementation:

• leader-and-follower, closest-replicas, and closest-adaptive are classified as ReplicaReadMixed
• ReplicaReadMixed will also attempt to read from learner replicas

This causes a behavior–documentation mismatch. Users who rely on the docs may assume that learners are never used under these modes, which can be surprising, especially when they deploy read-only / learner-only nodes.

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[gemini-code-assist]

Summary of Changes

Hello @King-Dylan, 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 enhances the accuracy of the TiDB documentation by clarifying the behavior of the tidb_replica_read system variable. Specifically, it updates the descriptions for the leader-and-follower and closest-replicas read modes to explicitly mention that learner replicas are also utilized for read operations, providing a more complete and precise understanding for users configuring replica read policies.

Highlights

• Documentation Clarity for Replica Read Modes: The documentation for the tidb_replica_read system variable has been updated to explicitly state that the leader-and-follower mode also considers learner replicas for read operations, ensuring a more accurate description of the system's behavior.
• Inclusion of Learner Replicas in Closest Replicas Mode: The description for the tidb_replica_read setting when set to closest-replicas has been clarified to include learner replicas as potential candidates for read operations within the same availability zone.

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[gemini-code-assist]

Code Review

This pull request clarifies that learner replicas are also considered for read operations when tidb_replica_read is set to leader-and-follower or closest-replicas. The changes are accurate and improve the documentation. I've added a couple of suggestions to improve conciseness and maintain a consistent voice throughout the list, in line with the documentation style guide.