tikv: update docs about in-memory pessimistic locks

[sticnarf]

Signed-off-by: Yilin Chen [email protected]

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[en-jin19]

@sticnarf could you please invite a tech reviewer? thank you!

[sticnarf]

@youjiali1995 I add one more section in the pessimistic transaction page. Please take another look. Thanks!

[sticnarf]

Could you take a look? @en-jin19

[you06]

@en-jin19 PTAL

[TomShawn]

LGTM

[TomShawn]

/remove-status LGT1
/status LGT2

[TomShawn]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: 1512067

[TomShawn]

@ran-huang 新增术语”内存悲观锁/in-memory pessimistic lock“：

zh：开启内存悲观锁功能后，悲观锁通常只会被存储在 Region leader 的内存中，而不会将锁持久化到磁盘，也不会通过 Raft 协议将锁同步到其他副本，因此可以大大降低悲观事务加锁的开销，提升悲观事务的吞吐并降低延迟。

en： When this feature is enabled, pessimistic locks are usually stored in the memory of the Region leader only, and are not persisted to disk or replicated through Raft to other replicas. This feature can greatly reduce the overhead of acquiring pessimistic locks and improve the throughput of pessimistic transactions.