Enrich instructions for string functions CHAR_LENGTH() and CHARACTER_LENGTH()

[qiancai]

This is a sub-issue of #15417.

This sub-issue aims to enrich instructions for string functions CHAR_LENGTH() and CHARACTER_LENGTH().

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 CHAR_LENGTH() and CHARACTER_LENGTH() #15483".

[PitifulPete]

/assign

[rpaik]

👋 @PitifulPete thanks for taking on this issue during TiDB Docs Dash! Just a friendly reminder to submit your PR by January 12 at 07:59 UTC.

Please reach out if you have any questions or need any help.

[qiancai]

@PitifulPete thank you so much for dedicating time to enrich instructions for string functions CHAR_LENGTH() and CHARACTER_LENGTH() ❤️. Since the PR #16057 for this issue has been merged, we will proceed to close this issue. If you come across any documentation issues in the future, please don't hesitate to create an issue or pull request in this repository 🚀.