langchain/chains/qa_with_sources/retrieval.py/RetrievalQAWithSourcesChain/_aget_docs
class RetrievalQAWithSourcesChain:    async def _aget_docs(self, inputs: Dict[str, Any]) -> List[Document]:
        question = inputs[self.question_key]
        docs = await self.retriever.aget_relevant_documents(question)
        return self._reduce_tokens_below_limit(docs)