fix chroma update_document to embed entire documents, fixes a characer-wise embedding bug

[cnellington]

Chroma update_document full document embeddings bugfix

Chroma update_document takes a single document, but treats the page_content sting of that document as a list when getting the new document embedding.

This is a two-fold problem, where the resulting embedding for the updated document is incorrect (it's only an embedding of the first character in the new page_content) and it calls the embedding function for every character in the new page_content string, using many tokens in the process.

Fixes #5582

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Tagging @dev2049 for vectorstore bugfix

[dev2049]

thanks @cnellington!