Issue on docs data-model/unique

- Page Link: https://doris.apache.org/zh-CN/docs/table-design/data-model/unique

- Version: 2.1

- Reason: 缺少了一条限制条件 “使用Unique表时，为了保证数据的唯一性，distributed by字段中不能包含非Key列”。案例：当我用下面这个SQL建表时，会报错 “1105: errCode = 2, detailMessage = Distribution column[c3] is not key column”

```sql
CREATE TABLE t1 (`c1` INT NULL, `c2` INT NULL, `c3` INT NULL)
ENGINE = OLAP
UNIQUE KEY(`c1`, c2)
DISTRIBUTED BY HASH(`c2`, c3)
BUCKETS 3
PROPERTIES (
"replication_allocation" = "tag.location.default: 1"
);
```

- May I contribute to this pull request? Our company has been frequently using Doris lately. If given the chance, I would also like to gradually become involved in the community.（如果确认该Issue的话，可以由我来提交PR吗，我很希望参与到doris社区当中）

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