Returns the contents of the buffer in text form.
If start is -1, then the entire buffer contents are returned. If start is not -1 but end is -1, then the contents are returned from start until the end of the buffer.
If flattened? is not #f, then flattened text is returned. See Getting Text for a discussion of flattened vs. non-flattened text.
If force-cr? is not #f and flattened? is not #f, then automatic carriage returns (from word-wrapping) are written into the return string as real carriage returns.