[fix] (docs) Fix Data type: string length and parameter description

[catpineapple]

Proposed changes

Issue Number: close #xxx

Problem summary

Describe your changes.

Checklist(Required)

1. Does it affect the original behavior:
   • [ ] Yes
   • [ ] No
   • [ ] I don't know
2. Has unit tests been added:
   • [ ] Yes
   • [ ] No
   • [ ] No Need
3. Has document been added or modified:
   • [ ] Yes
   • [ ] No
   • [ ] No Need
4. Does it need to update dependencies:
   • [ ] Yes
   • [ ] No
5. Are there any changes that cannot be rolled back:
   • [ ] Yes (If Yes, please explain WHY)
   • [ ] No

Further comments

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[github-actions[bot]]

PR approved by at least one committer and no changes requested.

[github-actions[bot]]

PR approved by anyone and no changes requested.

[morningman]

a soft limit of string type length, the hard limit is 2GB - 4, but if too long will cause very low performance, so we set a soft limit, default is 1MB, so the origin content is right?

Actually, we do not want user to save large string in string column. and it may change to a hard limit in future. So I think we should not say we support 2GB in document.

[ChPi]

@morningman 用户使用Doris作为唯一数仓，会存储比较大的数据，且从0.15版本开始使用。希望未来不要直接限死，还是大小可配置，文档可以指出数据太大导致的问题，让用户自己根据自己的集群业务情况限制大小

[morningman]

实际上目前的string不适合存储大字符串，所以这个可以理解为是当前Doris的功能限制，而不是一个可供用户选择的方案

[hf200012]

@catpineapple
please rebase