Added missed docstrings. Format docstrings to the consistent format
(used in the API Reference)
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Co-authored-by: ccurme <chester.curme@gmail.com>
0.2 is not a breaking release for core (but it is for langchain and
community)
To keep the core+langchain+community packages in sync at 0.2, we will
relax deps throughout the ecosystem to tolerate `langchain-core` 0.2