Context

As reported in https://github.com/langchain-ai/langchain-mongodb/issues/211, it can be easy to miss that the results are filtered by default by text_key. I think there are two things we could do while maintaining backwards compat:

• Add text_key to the example as suggested by evgeniishch
• Warn if no documents match the key, with the current value of text_key in the warning.

Definition of done

Address the usability of text_key

Pitfalls

Make sure that existing tests pass to preserve backwards compatibility.