iio: buffer-dmaengine: document iio_dmaengine_buffer_setup_ext

iio: buffer-dmaengine: document iio_dmaengine_buffer_setup_ext

The iio_dmaengine_buffer_setup_ext() function is public and should be
documented. Also, while touching this, fix the description of @dev in
related functions. @dev does not strictly have to be the parent of the
IIO device. It is only passed to dma_request_chan() so strictly
speaking, it can be any device that is a valid DMA channel consumer.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241115-dlech-mainline-spi-engine-offload-2-v5-9-bea815bd5ea5@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>