Update index documentation

[nb-park]

AnnotationFileCitation wrongly references a file index instead of a text index. To match the url citation better, I suggest """The index at which to insert the file citation in the message.""". A more verbose string could give an example of inserting the citation.

openai-python/src/openai/types/responses/response_output_text.py

Lines 28 to 29 in a825874

	index: int
	"""The index of the file in the list of files."""

My interpretation is founded in a discussion and experimental observation that index lines up with the end of a word in the output_text. Note that for a "manual" check account for new lines as a single character and be aware that a text editor may not have a zero-indexed character position.

See also the file search guide. It is slightly ambiguous until tested that it refers to to an insertion point and not the ___location within the cited document.

Making a further observation within the same file, AnnotationFilePath has the same documentation for index, but it isn't relevant to my use case so I'm not sure how accurate it is.