tikv: update default settings of server.grpc-concurrency and server.grpc-raft-conn-num.

[LykxSassinator]

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After tikv/tikv#18806, the default settings to server.grpc-concurrency and server.grpc-raft-conn-num has been modified, which is adaptive to the CPU cores by default.

This PR is introduced to update the descriptions to these two configurations.

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[ti-chi-bot]

Hi @LykxSassinator. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[Frank945946]

LGTM

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@Frank945946: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-07 06:15:47.464835627 +0000 UTC m=+769303.283144059: ☑️ agreed by hfxsd.
• 2026-01-12 05:49:37.220943939 +0000 UTC m=+250221.282808879: ☑️ agreed by qiancai.

[qiancai]

/ok-to-test

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21259.