MINOR: Add docs for StreamJoined changes (#9951)
MINOR: Add docs for StreamJoined changes (#9951)

Add docs for KIP-689.

Reviewers: Jim Galasyn <19bf0aecea2bae9799bd42acbde62714cabd7440@confluent.io>, Matthias J. Sax <matthias@confluent.io>
TiVo/kafka,guozhangwang/kafka,lindong28/kafka,lindong28/kafka,TiVo/kafka,guozhangwang/kafka,Chasego/kafka,lindong28/kafka,apache/kafka,apache/kafka,Chasego/kafka,guozhangwang/kafka,apache/kafka,TiVo/kafka,Chasego/kafka,Chasego/kafka,lindong28/kafka,TiVo/kafka,guozhangwang/kafka,apache/kafka