Enrich instructions for string functions LEFT() and LENGTH()

[qiancai]

This is a sub-issue of #15417.

This sub-issue aims to enrich instructions for string functions LEFT() and 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 LEFT() and LENGTH() #15491".

[takaidohigasi]

/assign

[rpaik]

Thanks @takaidohigasi for your contribution to the TiDB Docs Dash 🎉 Just a quick reminder to submit your PR by January 12 at 07:59 UTC (your local time) 😄

Please let us know if you have any questions.