DOC: Ineffective Configuration of MongoDBChatMessageHistory for Custom session_id Storage

[rongchenlin]

Checklist

• I added a very descriptive title to this issue.
• I included a link to the documentation page I am referring to (if applicable).

Issue with current documentation:

I tried to configure MongoDBChatMessageHistory using the code from the original documentation to store messages based on the passed session_id in MongoDB. However, this configuration did not take effect, and the session id in the database remained as 'test_session'. To resolve this issue, I found that when configuring MongoDBChatMessageHistory, it is necessary to set session_id=session_id instead of session_id=test_session.

pr: #18128

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