Use permalink

[zyguan]

Hi, @UNHNQ

I think it would be better to use permalink instead of normal link. eg.

-* [Request](https://github.com/pingcap/tidb/blob/source-code/kv/kv.go#L176): 请求的内容
+* [Request](https://github.com/pingcap/tidb/blob/45457ea8810ca7b835da4ba7f55d0eee02043ac5/kv/kv.go#L176): 请求的内容

BTW, something is missing here

用于生成全局唯一自增 ID 的模块，除了用于给每个表的自增 ID 之外，还用于生成???

[siyu-hu]

@shenli PTAL