Enrich instructions for string functions CONCAT() and CONCAT_WS()

[qiancai]

This is a sub-issue of #15417.

This sub-issue aims to enrich instructions for string functions CONCAT()) and CONCAT_WS().

Contribution guidelines
You can either enrich the instructions in English or Chinese.

1. Assign the issue to yourself by commenting /assign.
2. Submit a pull request (PR) to the TiDB docs repositories (either pingcap/docs in English or pingcap/docs-cn in Chinese, or both).
3. In the PR description, indicate which sub-issue this PR resolves, such as "Fix Enrich instructions for string functions CONCAT() and CONCAT_WS() #15484".

[Weaxs]

/assign