Add maxLengthTip to web and increase max-chunk-size

[xu-song]

Summary

1. Add maxLengthTip: Maximum chunk length is ambiguous. It refers to the num of tokens, which could be confused with string length.
2. Increase max-chunk-size: With default gpt2-tokenizer, 1000 tokens is roughly equivalent to 400 CJK characters. It is not enough in most cases.

Screenshots

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods