Return the processed flat text of a supported document (10-K, 10-Q, 8-K, Transcript, News Article). The response is a flat shape with no pagination and no section list; callers that need the section list or the recommended_call hint use the /metadata endpoint, and callers that need the original document file use GET /api/v2/documents/{id}. By default the response is capped at max_characters (100000) and truncated=true is set when the cap is hit. Pass full_content=true to bypass the cap. Unsupported filing types and PDFs return 400 with a reason; an unknown section name returns 400 with the available_sections list so the caller can recover.