Update the LangChain API doc with new index and query definitions 

For instance

• The API reference doc for for retriever has some old definition for filtering: https://api.python.langchain.com/en/latest/vectorstores/langchain_community.vectorstores.mongodb_atlas.MongoDBAtlasVectorSearch.html#langchain_community.vectorstores.mongodb_atlas.MongoDBAtlasVectorSearch.as_retriever

# Use a filter to only retrieve documents from a specific paper
docsearch.as_retriever(
    search_kwargs={'filter': {'paper_title':'GPT-4 Technical Report'}}
) 

The above filter syntax should follow the new version, which is 

qa_retriever = docsearch.as_retriever(
    search_kwargs={'filter': {'folderName': {'$eq': 'GPT-4'}}
)

It should also refer to creating a corresponding Vector Search Index.