Documentation: iio: Document high-speed DMABUF based API

Document the new DMABUF based API.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Co-developed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/20240620122726.41232-7-paul@crapouillou.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>