keywords: 优化建议，增加 TiDB 标识

[shawn0915]

Change Request

This repository is ONLY used to solve problems related to DOCS-CN. For other issues (TiDB, TiKV, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

https://github.com/pingcap/docs-cn/blob/master/keywords.md?plain=1#L63

建议增加 tidb 关键字标识，用以区分于mysql、iso标准支持的关键字，例如

TiDB 的保留字用 (T) 来标识。

对应 parser.y 这段

https://github.com/pingcap/tidb/blob/master/pkg/parser/parser.y#L805

/* The following tokens belong to TiDBKeyword. Notice: make sure these tokens are contained in TiDBKeyword. */

ref. https://asktug.com/t/topic/1021069

2. Describe your suggestion or addition.

3. Provide some reference materials (documents, websites, etc) if you could.

[Oreoxmt]

@tangenta Could you please take a look at this issue? Thanks!