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]

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[Frank945946]

LGTM

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