Enrich instructions for string functions HEX() and INSERT()

[qiancai]

This is a sub-issue of #15417.

This sub-issue aims to enrich instructions for string functions HEX() and INSERT().

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 HEX() and INSERT() #15488".

[Takashi-kun]

/assign