Issue on docs BloomFilter Index

- Docs Link: https://doris.apache.org/zh-CN/docs/table-design/index/bloomfilter

- Version: 2.1

- Reason: Lack of content in the introduction ”Bloom filter index only used in columns of UNIQUE_KEYS/DUP_KEYS table or key columns of AGG_KEYS table“, I think this restricted content is very important for users（Bloom Filter 索引仅能用于主键模型/明细模型表的列或聚合模型表的Key列，我认为这个提示对于用户来说非常重要，需要加到文档中）

- 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）

- ScreenShot:


Open the linked Version 2.1 BloomFilter index documentation page and compare its introduction with the requested restriction. Done means the introduction clearly states that Bloom filter indexes apply only to UNIQUE_KEYS/DUP_KEYS columns or AGG_KEYS key columns.

Written by the indexing model from the issue text.